Zhong Nanshan, the famous medical scientist, has devoted most of his life ________ useful ways in treating cough diseases.

My parents live in a small village. They always keep candles in the house_____there is a power out.

Howcan we reduce C02 in our home一It’seasy．________ offyour TV or computers when you’re not usingthem and you’ll greatly reduce it．

It was very kind of you to do the washing-up but you _____ it

He has _______ to Tibet many times. This time he _______ Tibet for two weeks.