Marianne is a professional chess player who hums audibly
while playing her matches, thereby distracting her opponents.
When ordered by chess officials to cease humming or else
be disqualified from professional chess, Marianne protested
the order. She argued that since she was unaware of her
humming, her humming was involuntary and that
therefore she should not be held responsible for it.

25. Which one of the following, if true, most undermines
Marianne's argument against the order?
(A) The officials of chess have little or no authority
to control the behavior of its professional
players outside of matches.
(B) Many of the customs of amateur chess matches
are not observed by professional chess players.
(C) Not all of a person's involuntary actions are
actions of which that person is unaware.
(D) A person who hums involuntarily can easily
learn to notice it and can thereby come to
control it.
(E) Not all of Marianne's opponents are distracted
by her humming during chess matches.

stuck between c and d, i guessed D correctly, however y the answer cant be C i cannot understand ????:(