题目

Although many Chinese students say that their knowledge of English grammar is good ,most would admit that their spoken English is poor .Whenever I speak to a Chinese student they always say, “My spoken English is poor.” However ,their spoken English does not have to remain “poor” ! I would like to suggest that there may be some reasons for their problems with spoken English.        First ,they fail to find suitable words to express themselves due to a limited vocabulary .Obviously the better answer is to expand their vocabulary. However ,you can speak with a limited vocabulary, （2004•上海模拟）如图，在梯形ABCD中，AD∥BC，∠A=90°，BC=DC，sinC=，BC=10，求AB的长．