To some people, happiness is __________ by family and friends, while to others, it means ___________ success.

A．being surrounded ; achieving

B．being surrounded ; to achieve

C．to surround ; achieving

D．surrounding ; to achieve

人们之所以采取某种行动，努力工作，是因为他觉得这种行为在一定概率上达到某种结果，并且这种结果可以带来他认为重要的报酬，这是（）。

A.期望理论 

B.公平理论 

C.强化理论 

D.双因素理论