In What Way Do Observations of Binary Star Systems Support

In what way do observations of binary star systems support the special theory of relativity?

A) Stars in binary systems often orbit at speeds close to the speed of light,allowing us to check whether time dilation really occurs.
B) Stars in binary systems are moving,so we can compare their moving masses to their rest masses.
C) The fact that the stars appear distinct supports the idea that the speed of light is unaffected by the motion of the stars.
D) Binary systems have two stars,and therefore give us twice the opportunity to test the theory than single star systems.