—Must I finish my homework now, Mom?—No, you _______.You can do it tomorrow.
A:can't
B:needn't
C:mustn't
己知反比例函数y= ,当1<x<3时,y的取值范围是( )。
“动圈式”麦克风的工作原理是 ____ (选填“电磁感应”、“磁场对电流有作用力”),其相当于 ____ (选填“发动机”、“电动机”)。