The word "they" in the passage refers to ______.

Supernovas are the most powerful and spectacular outbursts known in nature. What is called a Type II supernova is due to the collapse of a massive star, at least eight times as massive as the sun, that has used up its main nuclear fuel and produced a nickel-iron core. When this core can no longer support the pressure of the star’s outer layer, it collapses to form a neutron star of immense density. Over 2,500 million tons of neutron star packed into a matchbox. Its temperature is around 100,000 million degrees centigrade. Multitudes of neutrons are produced in the collapsed star, which pass directly through the star into space, and this release of neutrons causes the core to respond with a shock wave that moves outward. When it meets the material that is falling inward, the result is a catastrophic explosion. Sometimes most of the star’s material is blown away, leaving only a small, incredibly dense remnant that may be a neutron star or, in extreme cases, a black hole. A supernova is often more than 500 million times as luminous as the sun. A supernova remnant (SNR) may be detectable as a pulsar, an example of which is the Crab Nebula, known to be a remnant of the supernova observed in the year 1054. The 1987 supernova in the Large Cloud of Magellan had a low peak luminosity by supernova standards, only about 250 million times that of the sun. As its brightest the supernova shone as a star between magnitudes 2 and 3, even though it was 170,000 light-years away. 

What is the main topic of the passage?

According to the passage, which of the following is TRUE about the 1987 supernova?

The word “it” refers to __________. 

Traditionally in America, helping the poor was a matter for private charities or local governments. Arriving immigrants depended mainly on predecessors from their homeland to help them start a new life. In the late 19th and early 20th centuries, several European nations instituted public-welfare programs. But such a movement was slow to take hold in the United States because the rapid pace of industrialization and the ready availability of farmland seemed to confirm the belief that anyone who was willing to work could find a job. Most of the programs started during the Depression era were temporary relief measures, but one of the programs Social Security – has become an American institution. Paid for by deductions from the paychecks of working people, Social Security ensures that retired persons receive a modest monthly income and also provides unemployment insurance, disability insurance, and other assistance to those who need it. Social Security payments to retired persons can start at age 62, but many wait until age 65, when the payments are slightly higher. Recently, there has been concern that the Social Security fund may not have enough money to fulfill its obligations in the 21st century, when the population of elderly Americans is expected to increase dramatically. Policy makers have proposed various ways to make up the anticipated deficit, but a long-term solution is still being debated. In the years since Roosevelt, other American presidents have established assistance programs. These include Medicaid and Medicare; food stamps, certificates that people can use to purchase food; and public housing which is built at federal expense and made available to persons on low incomes. Needy Americans can also turn to sources other than the government for help. A broad spectrum of private charities and voluntary organizations is available. Volunteerism is on the rise in the United States, especially among retired persons. It is estimated that almost 50 percent of Americans over age 18 do volunteer work, and nearly 75 percent of U.S. households contribute money to charity. 

New immigrants to the U.S. could seek help from ______.

The word "frustrating" in paragraph 7 is closest in meaning to ______.

It can be inferred from the passage that the author ....

Most languages have several levels of vocabulary that may be used by the same speakers. In English, at least three have been identified and described. Standard usage includes those words and expressions understood, used, and accepted by a majority of the speakers of a language in any situation regardless of the levels of formality. As such, these words and expressions are well defined and listed in standard dictionaries. Colloquialisms, on the other hand, are familiar words and idioms that are understood by almost all speakers of a language and used in informal speech or writing, but not considered acceptable for more formal situations. Almost all idiomatic expressions are colloquial language. Slang, however, refers to words and expressions understood by a large number of speakers but not accepted as appropriate formal usage by the majority. Colloquial expressions and even slang may be found in standard dictionaries but will be so identified. Both Colloquial usage and slang are more common in speech than in writing. Colloquial speech often passes into standard speech. Some slang also passes into standard speech, but other slang expressions enjoy momentary popularity followed by obscurity. In some cases, the majority never accepts certain slang phrases but nevertheless retains them in their collective memories. Every generation seems to require its own set of words to describe familiar objects and events. It has been pointed out by a number of linguists that three cultural conditions are necessary for the creation of a large body of slang expressions. First, the introduction and acceptance of new objects and situations in the society; second, a diverse population with a large number of subgroups; third, association among the subgroups and the majority population. Finally, it is worth noting that the terms "standard", "colloquial" and "slang" exist only as abstract labels for scholars who study language. Only a tiny number of the speakers of any language will be aware that they are using colloquial or slang expressions. Most speakers of English will, during appropriate situations, select and use all three types of expressions. 

Which of the following is the main topic of the passage?