It is well known that knowledge is ____ treasure house, but practice is ___ key to it.

A．a; the

B．the; the

C．a; x

D．x; the

如图所示为录音机在工作时的示意图，轮子1是主动轮，轮子2为从动轮，轮1和轮2就是磁带盒内的两个转盘，空带一边半径为r₁=0.5 cm，满带一边半径为r₂=3 cm，已知主动轮转速不变，恒为n₁=36 r/min，试求：

(1)从动轮2的转速变化范围；

(2)磁带运动的速度变化范围。