请教OG154

154. 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
(E) and he set free the more than 500 slaves who were legally considered as

This sentence requires that the participial phrase setting free... connect to the gerund construction by filing a deed...; it was the filing of a deed that made possible the setting free .... Choices A and B establish this connection, but only A, the best choice, completes the participial phrase appropriately. In choices B and D the misconstructed phrases set[ting] free more than the 500 slaves ... mistakenly suggest that Carter set free slaves that were not his own. Choices C and D distort meaning by paralleling stunned and set free, as though
these were two separate and independent actions. E begins a second independent clause, which--though grammatically acceptable--again distorts the meaning. In choices B, C, and E, considered as is unidiomatic.

看不懂画线部分,为什么说b认为不是他的,后面不是有consider吗?consider as 怎么unidiomatic了?麻烦大家给解释一下,多谢