Correct Answer: C. were standing - was driving

Explanation:
This sentence describes two actions happening simultaneously in the past:

"We were standing in the rain" describes an ongoing action (Past Continuous).
"Sally's mother was driving past" describes another ongoing action occurring at the same time (Past Continuous).
Why not the other options?

A. stood - drove: Both actions are in the simple past, which implies that they happened one after another, not simultaneously.
B. were standing - drove: The first action is ongoing, but the second action ("drove") is a completed one, which doesn't match the context of simultaneous actions.
D. stood - was driving: The first action is completed, but the second action is ongoing, which is inconsistent with the idea of two ongoing actions at the same time.

Translation:
Khi chúng tôi đang đứng dưới mưa, mẹ của Sally đang lái xe ngang qua.