--- It’s difficult for the village children to cross the river for school.

--- I think a bridge _______ over the river.

A．should be built

B．is being built

C．has been built

D．was built

电阻R₁的阻值为5Ω，当电阻R₁与R₂以某种方式连接后接在电源两端时，R₁两端的电压为3V；当两电阻以另一种方式连接在同一电源两端时，R₁两端的电压为7V。请你在图中虚线框内画出电阻R₁与R₂的两种连接电路；