If you are traveling    the customs are really foreign to your own, please do as the Romans do.

A．in which

B．what

C．when

D．where

体积相同的NaCl、MgCl₂、AlCl₃溶液，沉淀其中的Cl^－，消耗等量的AgNO₃，三种溶液的物质的量浓度之比为　　　(    )

A．3∶2∶1

B．1∶2∶3

C．6∶3∶2

D．2∶3∶6