— I saw Mr. Green at school this morning.
— No, it _____ him. He flew to Canada last week.
A:can be
B:must be
C:can't be
如图所示,一带负电的滑块从粗糙斜面的顶端滑至底端时的速率为v,若加一个垂直纸面向外的匀强磁场,并保证滑块能滑至底端,则它滑至底端时的速率 ( )