148. Under the Safe Drinking Water Act, the Environ­mental Protection Agency is required either to approve individual state plans for controlling the discharge of wastes into underground water or
that they enforce their own plan for states without adequate regulations.
(A) that they enforce their
(B) for enforcing their
(C) they should enforce their
(D) it should enforce its
(E) to enforce its
答案是E (明顯

)
choices A, B, and C use plural pronouns (they and their) that have no grammatical referents.
但我對代名詞有問題 請問its 是代什麼阿???
還有他會什麼說沒有referent呢?