Câu hỏi:

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

Stars have been significant features in the design of many United States coins and their number has varied from one to foty-eight stars. Most of the coins issued from about 1799 to the early years of the twentieth century bore thirteen stars representing the thirteen original colonies.

  Curiously enough, the first American silver coins, issued in, 1794, had fifteen stars because by that time Vermont and Kentucky had joined the Union. At that time it was apparently the intention of mint officials to add a star for each new state. Following the admission of Tennessee in 1796, for example, some varieties of half dimes, dimes, and half-dollars were produced with sixteen stars.

          As more states were admitted to the Union, however, it quickly became apparent that this scheme would not prove practical and the coins from 1798 on were issued with only thirteen stars-one for each of the original colonies. Due to an error at the mint, one variety of the 1828 half cent was issued with only twelve stars. There is also a variety of the large cent with only 12 stars, but this is the result of a die break and is not a true error

What is the main topic of the passage?