In a Binary System, the First Star to Fill Its

Multiple Choice

In a binary system, the first star to fill its Roche lobe is usually a red giant. What can we say about the nature of the other star when this happens?

A) There is no correlation between the two stars, so the second star may be at any stage of evolution.
B) The two stars in a binary system must have the same mass, so the second star must be a red giant at this time also.
C) The two stars in a binary system are formed at the same time, so the other star must be a main-sequence star.
D) The two stars in a binary system are formed at the same time, so the other star is most likely still a protostar.