--- How many of you have solved the math problem?--- ________, the math problem is far too difficult for us to work out. .
A lot of supermarkets ________ in our city three years ago.
— In our city, people always see the blue sky this year. What do you think of it? — I think trees we plant, air our city will have.
______ are these bright purple shirts in front of the window made of?
She asked two persons the way to the bank, but of them could help her.
Tom Green ________ his parents every night even while he was traveling around China.