Which of the Following Would Not Be a Valid Reason

Which of the following would not be a valid reason for a substitution reaction being irreversible?

A) The reaction is highly exothermic.
B) The reaction requires a catalyst to proceed.
C) One product is a gas that bubbles out of the reaction medium.
D) One product crystallizes out of the reaction medium.
E) One reactant is present in a very large quantity relative to the other.