求助一道题！！

1.       In 1791 Robert Carter III, one of the wealthiest plantation owners in Virginia, stunned his family, friends, and neighbors by filing a deed of emancipation, setting free the more than 500 slaves who were legally considered his property.

(A) setting free the more than 500 slaves who were legally considered

(B) setting free more than the 500 slaves legally considered as

(C) and set free more than 500 slaves, who were legally considered as

(D) and set free more than the 500 slaves who were legally considered（A）

(E) and he set free the more than 500 slaves who were legally considered as

不是很明白 为什么要选A，并且不知道consider as 与 consider(to be) 有什么区别？

还请大家指教

THANK YOU