题目

如图4-9所示，一个有弹性的金属环被一根橡皮绳吊于通电直导线的下方，当通电直导线中的电流I增大时，圆环的面积S和橡皮绳的长度L将（    ） 图4-9A.S增大，L变短                            B.S增大，L变长C.S减小，L变短                            D.S减小，L变长 答案：思路解析：通电直导线周围的磁场离导线越远就越弱，当直导线中的电流增大时，通过金属环的磁通量增加，产生的感应电流必然要阻碍磁通量的增加.由于金属环有弹性，金属环面积缩小，同时橡皮绳拉长，金属环远离直导线.正确选项为D.答案：D The photo _____ taken by my brother last week is very nice.A. which were B. that isC. that were D. which was