如图，电路中三个电阻R_l、R₂和R₃的阻值分别为1Ω、2Ω和4Ω．当电键S₁断开、S₂闭合时，电源输功率为1W；当S₁闭合、S₂断开时，电源输出功率也为1W．则电源电动势为______V，内阻为______Ω．

As is known to all, Montreal is the second largest French-speaking city in the world, ______．

A．Paris is the largest

B．Paris the largest

C．Paris to be the largest

D．Paris be the largest