In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").

Fact1: contact between rocks over long periods of time causes rocks to smooth, Fact2: Rocks on the streambed are subrounded and fairly smooth., Question: Contact between rocks over long periods of time causes rocks to be what? (A) coiled (B) homogenous (C) inactive (D) subrounded (E) dormant state (F) barrel-shaped (G) erosion (H) threatened
D