This is the complete sentence:
We were planting flowers in the school garden when it started to rain.I was able to find the complete exercise with its options to choose from online, which are:
were planting plant are planting have planted"Were planting" is the only one that is gramatically correct to complete the sentence given, because the sentence says that we were doing something when something else happened, so you need to start with the past continuons.
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