Choose the correct phrase to complete the traveler's miserable (but funny) diary entry about a camping trip gone wrong.

When we finally managed to pitch the tent at midnight, we were completely exhausted and covered in thick mud. It _____ heavily since we had left the highway five hours earlier, turning the trail into a swamp.

The correct answer is had been raining.

The Past Perfect Continuous is required here because the sentence describes an action that started in the past and continued up to another point in the past (midnight).

The key indicator is the word "since" combined with a past reference point ("since we had left the highway"). Was raining is incorrect because "since" requires a perfect tense to measure the duration up to that past moment. Has been raining is the present perfect continuous and does not fit a story told entirely in the past tense.