—Who has made a mess in my room?—Who else       it but your naughty son?

If I ______ about this computer program, a hage amount of time and energy would have been saved.

Had I known her name，I________.

.He has just arrived, but he talks as if he _____ all about that.

—Do you think he will do me a favor?—As far as I know，he is the last one to help others.He_______be prepared to give you a hand，though.