题目

. It was _____ his mother came in _____ to prepare his lessons. A. not until; did the boy begin                 B. until; that the boy didn’t begin C. until; did the boy not begin                 D. not until; that the boy began 答案：A关于磁场和磁感线的描述，正确的说法是：（    ）A.磁感线从磁体的N极出发，终止于S极B.磁场的方向就是通电导体在磁场中某点受磁场作用力的方向C.沿磁感线方向，磁场逐渐减弱D.在磁场强的地方同一通电导体受的安培力可能比在磁场弱的地方受的安培力小