Most oxygen atoms have eight neutrons, but a small amount have nine or ten.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from questions from 25 to 29. 

NUCLEAR WASTE

One major problem for nuclear power is the waste it (25) ______. A typical nuclear reactor needs to get rid of 20 tons of spent fuel per year. The radioactivity in this waste takes 1000 years to diminish to negligible levels other dangerous contents include longer-lived fission products and uranium and plutonium, the 239 isotope of (26) _____ takes more than 150,000 years to decay to negligible amounts. It is important to keep this material away from people. To that end, the spent fuel initially goes into a pool at the reactor, (27) _____ it is kept beneath 5 metres of cooling water that blocks its radiation. After some years, the fuel can be removed and put into dry cask storage which, as the phrase (28) _____, means it is put in big drums at the plant. Two such casks can hold one year's spent fuel safely for 100 years. After that, the casks should ideally be put into a mined geological repository - entombed in deep underground tunnels in a geologically stable (29) _____. 

My brother has given up smoking. He used to _____ 20 cigarettes a day.

Nam passed the exam easily _____ it was very difficult. 

Sue and Mira are talking about the use of mobile phone in class. 

- Sue: "Students should not be allowed to use mobile phone in class." 

- Mira: “______________. This will distract them from studying.” 

He can't find his bike anywhere. Someone _____ it.