2. 考研
  3. ______a new technique, the yields as a whole increased by 20 percent.

______a new technique, the yields as a whole increased by 20 percent.

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问题 ______a new technique, the yields as a whole increased by 20 percent.
选项 A、Working out
B、Having worked out
C、Having been worked out
D、To have been worked out
答案C
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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