a good insulator(绝缘体) rubber is often used in cables.A. Having beenB. To beC. BeD. Bei

问题补充：

a good insulator(绝缘体) , rubber is often used in cables.

A. Having been

B. To be

C. Be

D. Being

答案：

A. Having been

B. To be

C. Be

(答案→)D. Being

解析：现在分词短语表原因，状态等；不定时表示将来，目的。