The integer x is divisible by both 7 and 21

The integer x is divisible by both 7 and 21. Which of the following must be an integer?

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 #17542