请教补充250-209

请教补充250-209

209.       Television programs developed in conjunction with the marketing of toys, which was once prohibited by federal regulations, are thriving in the free market conditions permitted by the current Federal Communications Commission.

(A) Television programs developed in conjunction with the marketing of toys, which was once prohibited by federal regulations, are

(B) Television programs developed in conjunction with the marketing of toys, a practice that federal regulations once prohibited, is

(C) Developing television programs in conjunction with the marketing of toys, as once prohibited by federal regulations, is

(D) Federal regulations once prohibited developing television programs in conjunction with the marketing of toys, but they are

(E) Federal regulations once prohibited developing television programs in conjunction with the marketing of toys, but such programs are

答案是E，但是请问B错在那里呢？是因为thriving的不能是一种practice吗？但是好像也说的通吗。。。

我觉得B的问题是后面应该用are 而不是is

practice应该是前边整个句子所指的行为，做同位语成分，这种形式在大全里很常见阿,比如后面212那题

        Following the destruction of the space shuttle Challenger, investigators concluded that many key people employed by the National Aeronautics and Space Administration and its contractors work an excessive amount ofovertime, a practice that can, potentially, cause errors in judgment.

不知道除此之外B还有什么错？