Bunuel wrote:
To protect the depleting supply of migratory bird resting and nesting areas, state agencies are using water resources twice as fast as their natural rate, consulting with experts prior to the construction of new dams, and studying the effects of pollution on migratory patterns.
(A) their natural rate, consulting with experts
(B) their natural rate, experts consulted with
(C) naturally using them, consulting with experts
(D) the previous rate, consulting with experts
(E) the previous rate, a result of experts consulted with
OFFICIAL EXPLANATION
D
First, notice that the sentence contains a list. The state agencies are "using", "consulting", and "studying". "Using" and "studying" can't be changed, so "consulting" (which is parallel with "using" and "studying") must remain the same. That eliminates choices (B) and (E). The three remaining choices differ in the first part of the underlined portion.
Both (A) and (C) use a pronoun, "their" or "them". While it is clear from context that the pronoun refers to the water resources, the GMAT holds you to a higher grammatical standard. In this part of the sentence, there are two plural nouns, "state agencies" and "water resources". Technically, the pronoun could refer to either of those, so "the previous rate" - which could only refer to the rate that water resources are being used - must be correct.