For many years people electric ears. However making them has been more difficult than p

问题补充：

For many years, people electric ears. However, making them has been more difficult than predicted.

A. had dreamed of

B. have dreamed of

C. dreamed of

D. dream of

答案：

A. had dreamed of

(答案→)B. have dreamed of

C. dreamed of

D. dream of

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