一个教师可讲授多门课程，一门课程可由多个教师讲授。则实体教师和课程间的联系是
A．1:1联系B．1:m联系C．m:1联系D．m:n联系
参考答案：D
由于受酸侵蚀，而引起牙齿硬组织进行性丧失的酸蚀症的病因有（）
A.饮食酸
B.无机酸如：盐酸、硝酸和硫酸等
C.酸性药物
D.胃酸
E.以上均正确
We ______ a pleasant journey but for the rain. [ ]
A. would have
B. will have
C. had had
D. would have had
