C

@heidiz, Happy to help. Let's look at (C). (C) says, "Although no firm conclusions regarding them have yet been reached, both have been extensively investigated." the problem with (C) is that although the author does investigate the two theories a bit, we can't make the claim that the theories have been "extensively investigated" because we simply don't have sufficient support for that claim. Thus, we can get rid of (C). Hope this helps. Let us know if you have any other questions!