荧光抗体染色技术中，特异性最高，非特异性染色最低的方法是（）

A.直接法

B.间接法

C.补体结合法

D.双标记法

E.多标记法

The fire brigade arrived two hours later after the alarm was sounded,______it was too late to save much of the building.

A．then

B．when

C．which

D．though