Since 1986, when the Department of Labor began to allow investment officers' fees to be based on how the funds they manage perform, several corporations began paying their investment advisers a small basic fee, with a contract promising higher fees if the managers perform well.
(A) investment officers’ fees to be based on how the funds they manage perform, several corporations began
(B) investment officers’ fees to be based on the performance of the funds they manage, several corporations began
(C) that fees of investment officers be based on how the funds they manage perform, several corporations have begun
(D) fees of investment officers to be based on the performance of the funds they manage, several corporations have begun
(E) that investment officers' fees be based on the performance of the funds they manage, several corporations began
解释中说ABE三个选项中的they都指代不明，这是什么意思呢？they在这三项中可以指代那些名词呢？有关指代的语法规则是什么呢，请大牛指教。为什么正确选项中的they指代就明确呢？ | 
