过去进行时 — 英语填空题
题目
根据句意，用括号内动词的适当形式填空：Last night, while we ________ (wait) for the heavy rain to stop, my little brother suddenly ________ (drop) his glass of milk on the floor. So he ________ (clean) it up at that time and didn't finish his homework until 10 o'clock.