13. A new technique_________, the yields as a whole increased by 20 percent.

A.working out

B.having worked out

C.having been worked out

D.to have been worked out

C


解析:

該題考查現(xiàn)在分詞短語作狀語的用法，由題意知“新技術(shù)應(yīng)是被鉆研出”，此處應(yīng)用其被動語態(tài)形式，即“having been+過去分詞”。?

答案：C?

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