系统在持续的外界干扰力的作用下被迫产生的振动称为（）

A．自由振动

B．受迫振动

C．胁迫振动

D．干扰振动

We will spend our summer holiday in Guilin, but we haven’t decided______________.

A．what to buy

B．where to go

C．how to go

D．which to choose