Technically, charring of bones takes a loooong time and does not happen unless some antelope accidentally got caught in a forest fire (This is ext. knowledge obtained from loads of crime-related programs on Discovery). Hence the fire that charred those bones should have been controlled by man.
Coming to the question otherwise, if we refute A, we don't get anything concrete. The main point is not that man cooked the meat, but that the fire used was a controlled one. That makes B a better choice than A
Am I making ANY sense? This is how I answer a good number of questions and also get them right