Working hard is not only a of great success but it is among the essential requirement.A.

问题补充：

Working hard is not only a of great success, but it is among the essential requirement.

A. sign

B. singal

C. guarantee

D. expectation

答案：

A. sign

B. singal

(答案→)C. guarantee

D. expectation

解析：句意：勤奋工作不仅仅是巨大成功的保证，它还是基本要求之一。A. sign符号、标志；B. singal 信号；C. guarantee保证；D. expectation期待。由句意可知选C。

考点: 考查名词的用法。