By What Mechanism Does a Second Male Mating with a Female

By what mechanism does a second male mating with a female gain an advantage in sperm competition in some insects?

A) The second male's semen has chemicals that kill the first male's sperm.
B) The accessory fluids from the second male's semen can displace stored sperm from competing males.
C) The second male will have larger testes and so produce larger numbers of sperm.
D) The second male is larger than the first male.