The National Labor Relations Act expressly forbids unions from engaging in secondary boycotts against companies not directly involved in a labor dispute.
(A) unions from engaging in
(B) the engagement by unions of
(C) unions to engage in
(D) unions from becoming engaged with(C)
(E) that unions engage upon
我查过longman,forbid sb/sth to do/from doing ,是习惯用法,但区别在哪里呢?为什么要选C呢?