— 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
抗日战争时期,我国著名京剧表演艺术家梅兰芳留了胡须,拒绝为日本侵略者演出。梅兰芳的做法体现出( )
通电螺线管中的电流方向如图所示,由此可以判断出通电螺线管的左端是 ____极.(选填“N”或“S”)