题目

若直线l1到直线l2的角为θ1,l2到l1的角为θ2,则cos(+)的值为(    )A.           B.1                   C.0                    D.不能确定 答案：C解析：∵θ1+θ2=π,∴+=.∴cos(+)=0.He had learned________such things. So he refused________. A. to do; being taught to 　　　　　　　　    B. doing; to be taught to C. to do; to be taught to 　　　　　　　　　　　　   D. doing; being taught to