Câu hỏi:

Read the following passage and mark the letter A, B, C, D on your answer sheet to indicate the correct answer to each of the questions from 35 to 41.

Stars have been significant features in the design of many United States coins and their number has varied from one to forty-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 has 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 starts.

As more states were admitted to the Union, however, it quickly became apparent that this scheme would not prove practical and the coins from A798 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 A828 half cent was issued with only twelve stars. There is also a variety of the large cent with only A2 stars, but this is the result of a die break and is not a true error.

The word “their” in line 1 refers to _______