The physics teacher once said that as __________ well-known, thunders __________ make electricity.
_____ has already been pointed out, grammar is not a set of dead rules.
.---There is a lot of smoke coming out of the teaching building. ---Really? It _____be a fire, most probably.
–Did you find your new dictionary? ---Yes. I found it ____I had left it.
–Will you ______me ____out this problem myself? ---OK.
–When did the concert_____? ---Oh, two days ago.