B. It modifies the root word in a negative way.
Even though “anti,” by definition, can mean “the opposite of” something, it does not necessarily always make the word into an antonym. Because of the negative connotation of “anti,” however, what does happen is that the word is modified in a negative manner. For instance, the opposite of “hero” is “villain.” To put “anti” in front of “hero” gives you “antihero,” which is an unlikely hero of undesirable, unconventional attributes, which, as you can see is still a “hero,” not its opposite, but still negative.
