[求助]PREP1-104

104. (30075-!-item-!-188;#058&004088)
In Britain, "pig" refers to any member of the class of domestic swine, but the United States uses the
term when referring only to younger swine not yet ready for market and weighing less than 82
kilograms (180 pounds).
(A) the United States uses the term when referring
(B) the United States term refers
(C) in the United States the term refers
(D) in the United States they use the term as it refers
(E) it is used in the United States when referring

答案是C。

我对这个答案没有疑惑，只是有些不明白这里的介词短语的位置。

In Britain 介词短语放在句首，后面跟了逗号。但是 in the United States 也是在句首啊，为什么后面没有逗号呢？

哪位NN好心帮我解答一下吧

先谢谢了。