Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem ...

Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
A) A star with a mass of 3.7 MSun is too big to be a main sequence star.
B) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
C) It doesn't make sense to find a subgiant in a binary star system.
D) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main sequence stars or both be subgiants.