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.

plan for 是固定用法: 为....作计划,打算

何况,你细读几遍,..is required..to approve individual state plans for controlling the discharge ...

感觉远好于...is required..to approve individual state plans to control the discharge ...