ANSWER
Knowledge of adverb clauses
Topic: It started to rain. We ran as fast as we could to the hideout.
C. It started to rain, at which point we ran as fast as we could to get to the shelter.
Consider the remaining answers:
A. Even though it was raining, we ran as fast as we could to get to the shelter
B. It started to rain but we ran as fast as we could to get to the shelter
(This answer sounds reasonable, but in reality is incorrect, because the two incidents “it started to rain” and “we ran as fast as we could to get to the shelter” are completely two independent actions. It is not contradictory to use the conjunction “but” here; instead, the conjunction “so” can be used to indicate a cause-effect relationship.)
D. Unless it was raining, we ran as fast as we could to get to the shelter
Structure to note:
+ Start to do sth: start to do
+ At one (that/which/….) point: at some time, at a certain time