Fill in the blanks with the most appropriate words. When he ___ in London, it ______ heavily.

The correct answer is: C. arrived / was raining

The sentence is in the past tense, so we need to use a past tense verb for both the first and second blanks. The verb “arrive” is in the simple past tense, so we need to use the past participle “arrived” in the second blank. The verb “rain” is an irregular verb, so the past tense is “rained.”

Option A is incorrect because the verb “arrives” is in the present tense. Option B is incorrect because the verb “rains” is in the present tense. Option D is incorrect because the verb “will arrive” is in the future tense.