What's wrong with the shirt?
A:It is too large.
B:It is too small.
C:It is too thick.
某同学将一把长20cm的塑料尺用手按在桌面上,另一只手拨动尺子使之振动发声(如图所示),三次钢尺露出桌面的长度分别为12cm,9cm,6cm,比较三次发声,音调最高的是( )