请教一下 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

答案是C

弱弱的问D为啥不对呢？

谢谢

但but不是可以作介词吗？

but:

prep.
除...以外
conj.
而是, 但是
adv.
几乎, 仅仅

根据本题句意，bu此处是但是的意思