__________ you understand the rule, you will have no further difficulty.
____ computers are of great benefit to the development of science and technology.
____ puzzled the mother most was ____the son would never agree with her.
— ____ in this street? — I think Mr. Black is the richest man.
I haven’t found my watch yet; in fact, I’m not sure ____ I could have done with it.
I was advised to arrange for insurance ____ I needed medical treatment.