包材拉不动时，首先应检查（）是否工作。
参考答案：纸路各辊轮各送纸马达
如图15-2-12所示，导线AB竖直放置，通电导线框abcd可自由移动.当AB中通以图示电流时，导线框怎样运动？
图15-2-12
Unlike watching TV, reading is a highly active process________it requires attention as well as
memory and imagination.[ ]
A. until
B. but
C. unless
D. for
