21. Scientists are convincedthe positive effect of laughter_____physical and mental health.
A. of, at B. by, in C. of, on D. on, at
22. Alcohol is also classified as a downer, and though it is legal, it can damage the liver and do great harm if ________ in large quantities.
A. consume B. to consume C. consuming D. consumed
23. Betty shows great interest in this poem now, but she thought it boring _______ she read it.
A. at the first time B. the first time C. for the first time D. at first
24. The number 2008 is a special number, ________ I think, that will be remembered by the Chinese forever.
A. which B. what C. one D. it