"Rains" fall from the sky.
The plural of rain in English sounds wrong to me. However, I'm German and not a native speaker. This seems to me like a technical translation, that is structurally equivalent but not idiomatically correct.
Perhaps the course designers wanted to check that we can put imber into the plural. I'm quite sure I've seen it in singular in quite a few textbooks. Is it common to refer to imbres, not imber?
What's the difference between "de" and "a/ab"? is it possible to say "Imbres a caelo descendunt" ?