1 . It is worth ________ to stay in touch with your friends, for you can turn to ________ for help when in trouble.

A．that; themselves

B．it; themselves

C．those; them

D．it; them

2 . 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

3 . Tell_________about the Great Wall please.

A．I

B．my

C．me

D．mine

4 . 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

5 . California is home to 4,426 species of plants, and nearly half of ________ are found nowhere else.

A．which

B．them

C．what

D．that