The workers in HuaShan Mountains have to     rubbish to keep the mountains clean.

–Attention, please. The flight has to be_______ because of the heavy rain.

---Miss Green’s new dress looks smart.      ---I wonder what it was        of.

―Do you prefer spring ________ summer?―________, because it’s warm and flowers start to come out．

—How long have you________?—Since two years ago.