1 . In the old days, stock brokers earned commissions every time________ bought or sold a stock on their client’s behalf.

A．when they

B．they

C．/

D．that

2 . Nowadays many old people raise dogs as pets to keep ________company so that they will no longer feel lonely.

A．them

B．themselves

C．it

D．itself

3 . It is probably impossible for you to understand anything new without the mind referring to the basic knowledge _______ has already obtained.

A．it

B．/

C．that

D．which