Algol Consist of a 3

Multiple Choice

Algol consist of a 3.7 MSᵤn main-sequence star and a 0.8 MSᵤn subgiant.Why does this seem surprising,at least at first?

A) 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.
B) It doesn't make sense to find a subgiant in a binary star system.
C) The two stars should be the same age,so we'd expect the subgiant to be more massive than the main-sequence star.
D) A star with a mass of 3.7 MSᵤn is too big to be a main-sequence star.