Tom’s mother kept telling him that he should work harder, but _________ didn’t help.

A．he

B．which

C．she

D．it

R₁＝R₂＝R₃。开关S₁、S₂的状态如图所示，此时电路中的R₂、R₃是            联的；要使R₁、R₂串联在电路中，则开关S₁、S₂的状态是                   。