单元顶级和多元顶级之间的真正差异可能在于( )。

A．衡量群落最稳定性的时间因素

B．衡量群落相对稳定性的时间因素

C．衡量群落最稳定性的结构因素

D．衡量群落相对稳定性的结构因素

When starting CPR on a drowning victim，you should（）.

A.start chest compressions before the victim is removed from the water

B.drain water from the lungs before ventilating

C.begin mouth-to-mouth ventilations as soon as possible

D.do not tilt the head back since it may cause vomiting