题目

The main cause of HIV / AIDS infection in China is drug abuse,  ______ is very serious in Yunnan. A. what                            B. who            C. that     D. which   答案：D考查定语从句。which引导一个非限制性定语从句。原句出自China Daily。点M是半径为5的⊙O内一点，且OM=4，在过M所有⊙O的弦中，弦长为整数的弦的条数为    ．