The Sanlu Milk Powder event is a clear indication that food safety is a big problem; nobody can __   _ all foods are secure to us, but we are sure that the situation is turning for the better.

A．acknowledge

B．guarantee

C．identity

D．clarify

理论塔板数反映了（）。

A、分离度

B、分配系数

C、保留值

D、柱的效能