. different kinds of pianos, the workers farther improved their quality.
A. To produce
B. Being produced
C. Produced
D. Having produced
网友回答
A. To produce
B. Being produced
C. Produced
(答案→)D. Having produced
解析：考察非谓语动词.句意:生产过各种不同的钢琴,工人们进一步的 提高了他们的质量. 现在分词表示正在进行的伴随性动作,如果发生在谓语动词之前,则要用完成时态的现在分词,故选D.