---What _______ the number of the students in your school? ---About two thousand.A number of them _______ from the countryside. A. is; are B. is; is C. are; is D. are; are 答案:A一台理想变压器的原、副线圈匝数比为15∶3,这台变压器工作时,原线圈上的频率和副线圈上的频率之比等于A.1∶1B.15∶3C.3∶15D.225∶9