One Star in a Binary Will Almost Always Become a Red

One star in a binary will almost always become a red giant before the other because

A) one star is always larger than the other.
B) binaries always have one star twice as massive as the other.
C) small differences in main-sequence masses yield large differences in main-sequence ages.
D) the more massive binary star always gets more mass from the less massive binary star when both are main-sequence stars.