—How unhappy your roommate looks!—Yes, but he's not willing to tell me what it is ________ is troubling him.
Tom tried to __________ in German.
I wonder how many years ago ___________.
He said that he would write to us but so far we ___________ from him.
__________ good exercise.
I don’t know who invented __________ telescope, but I think it is __________ most useful invention.