[問題]OG(VR)80

[poo]

80．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

Ans:(E)
請問whereby的用法
為什麼 (D)whereby introduces a sentence fragment？
謝謝

[吳]

if D is correct, which one(sentence) is the main sentence?