Not everyone has realized what important part the balance of nature plays in their life.A.

问题补充：

Not everyone has realized what important part the balance of nature plays in their life.

A. an；the

B. /；the

C. an；/

D. the；/

答案：

A. an；the

B. /；the

(答案→)C. an；/

D. the；/

解析：考查冠词及短语的用法。第二空nature“自然”时前面不需用冠词，类似的还有space“太空”；man“人类”。第一空是play an important part in their life短语，用了what引导的感叹句，正常语序应是the balance of nature plays an important part in their life，被感叹的是名词part所以用what来引导，故选C。用how来感叹形容词或副词。