If positive integer n is divisible by both 6 and 21, then…

If positive integer n is divisible by both 6 and 21, then n must be divisible by which of the following?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #9393