12. GWD26-Q12 ??

12. GWD26-Q12 ??

The most vexing problem faced by researchers exploring wind-powered
        generation of electricity is achieving a constant flow of power
            from an unpredictable natural source that meets demand, but so the flow does not overload electrical grids with sudden
        voltage increases

A.      achieving a constant flow of power from an unpredictable natural source that meets demand, but so the flow does not overload

B.      achieving a constant flow of power from an
        unpredictable natural source, which meets demand but without overloading

C.      how to achieve a constant flow of power
        from an unpredictable natural source, a flow that meets demand but does not overload

D.      how to achieve a constant flow of power
        from an unpredictable natural source, but
        a flow that meets demand without overloading

E.       how a constant flow of power from an unpredictable natural source can be achieved, which
        meets demand but does not overload

   答案C

your question?

A.      achieving a constant flow of power from an unpredictable natural source that meets demand, but so the flow does not overload

so在这里指代不清，排除A

B.      achieving a constant flow of power from an
        unpredictable natural source, which meets demand but without overloading

but应该连接平行成分，meets VS without 不对应，排除B

C.      how to achieve a constant flow of power
        from an unpredictable natural source, a flow that meets demand but does not overload

正确

D.      how to achieve a constant flow of power
        from an unpredictable natural source, but
        a flow that meets demand without overloading

but中应该省略主语，而且but在这里逻辑不对

   谢谢！