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
E, the best choice, is the only one that maintains grammatical parallelism by using an infinitive--to enforce--to complete the construction either to approve ... or.... All of the other choices offer syntactic structures that are not parallel to the infinitive phrase to approve. In addition, choices A, B, and C use plural pronouns (they and their) that have no grammatical referents.
My Question is: why use "controlling", not "to control"? Because the sentence has a previous “to do” or other reason? Usually we use “do sth to do” to express the purpose of verb.
EPA is required either to approve individual state plans for controlling the discharge of waste into underground water.
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