Someone is ringing the doorbell. Go and see _____.[ ]

He found very difficult the math problem.

The window ____ by Tom yesterday. Mr. White is repairing it now.

The speech by themayor of Shanghai before the final voting for EXPO 2010 is strongly impressed_______ my memory.

How much did you spend on the new mobile phone—I _____ it would cost me 500 dollars, but I was so lucky to get it at a 15 percent discount.