[SC. ] GWD26-Q12 be動詞之後接名詞或是形容詞only?

[kernnel (沒有你的日記)]

12. GWD26-Q12
The most vexing problem faced by researchers exploring wind-powered generation

of electricity is achieving a constant flow of power from
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an unpredictable natural source that meets demand, but so the flow does not
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overload electrical grids with sudden voltage increases
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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

想請問  我的觀念是be動詞之後只能接名詞或是形容詞 不能接副詞子句或是副詞片語
請問這邊的how to achieve....不算是副詞片語嗎? 還是這邊有另外的詞性我沒弄清楚?
感謝.

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推 imdodoim:這裡的how to ... natural source 應該是名詞子句