GMAT Verbal Sentence Correction - ChaseDream

(11) Under a provision of the Constitution that was never applied, Congress has been required to call a convention for considering possible amendments to the document when formally asked to do it by the legislatures of two-thirds of the states.

(A) was never applied, Congress has been required to call a convention for considering possible amendments to the document when formally asked to do it

(B) was never applied, there has been a requirement that Congress call a convention for consideration of possible amendments to the document when asked to do it formally

(C) was never applied, whereby Congress is required to call a convention for considering possible amendments to the document when asked to do it formally

(D) has never been applied, whereby Congress is required to call a convention to consider possible amendments to the document when formally asked to do so

(E) has never been applied, Congress is required to call a convention to consider possible amendments to the document when formally asked to do so

Q:这个题目的D选项，GMAT官方的分析说是因为后半句是一个dependent clause，即不完整的句子，有哪位大神能给我解释一下为什么这句话是dependent clause,另外怎么判断是不是完整句子呢，谢谢啦！