求教　白勇　四章９题

since 1986, when the department of labor began to allow fees of investment officers to be  based on the performances of the funds they manage, several corporations have begun paying their investment advisers a small basic fee, with a contract promising higher fees if the managers perform well.

文中已经是正确答案，但是我对划线部分不理解．　officers是人，为什么要用of来表所有格啊？不是很应该用officers'吗？

还有就是begun，我觉得它是不可以持续动词啊，为什么可以用在完成时里面呢？

谢谢赐教！

我觉得，investment officer这个词组有点长，用所有格形式不如用of形式好，而且lz从哪里看到过"人"只能用所有格形式来表示“……的”呢？

还有就是动词是不是持续性动词都可以用完成时的，而且句首有提示“since 1986”，所以主句必须用完成时。