单选题
1.
______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
【正确答案】
C
【答案解析】
现在分词的完成时表示分词的动作发生在主句谓语动作之前，由“having+过去分词”构成。
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