____ you first met Tom. 

A． where was it that

B．that was it where

C．where it was that

D．that was it where

把0.6molX气体和0.4molY气体混合于2L容器中，使它们发生如下反应：3X（气）+Y（气）=nZ（气）+2W（气）．5min末已生成0.2molW，若测知以Z浓度变化来表示的化学反应平均速率为0.01mol•L^-1•min^-1，则上述反应中Z气体的计量系数 n的值是（　　）

A．1

B．2

C．3

D．4