Explanation
Correct answer: A.
Choice A is the best answer. The noun that goes with "expand and contract" is "rivers," a plural noun. "They" is a third-person plural pronoun, so it can correctly stand in for "rivers."
Why the other choices are wrong
Choice B
Choice B is incorrect. This choice creates a pronoun-antecedent agreement error. "One" is a singular pronoun, but the noun that goes with "expand and contract" is "rivers," a plural noun.
Choice C
Choice C is incorrect. This choice creates a pronoun-antecedent agreement error. "It" is a singular pronoun, but the noun that goes with "expand and contract" is "rivers," a plural noun.
Choice D
Choice D is incorrect. This choice creates a pronoun-antecedent agreement error. The noun that goes with "expand and contract" is the plural noun "rivers." Rivers are not people, so "we" can’t be used to stand in for it.