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？ | 
