The old man sat in front of the television every evening, happy ___ anything that happened to be on.

[ ]

A. to watch

B. watching  

C. watched    

D. to have watched

如图所示，在磁感强度为B的匀强磁场中，有一个边长为L的正六边形线圈abcdef，线圈平面垂直于磁场方向，则穿过线圈的磁通量大小为______；若去掉其中的一条边ab，且通有图示顺时针方向的电流，电流强度大小为I，则剩余的五条边所受安培力的大小为______（线圈不发生形变）．