Think about the Earth and Mars in their orbit around the Sun. Where is the center of mass located for this three body system?
a) It is closer to the Earth, than it is to either the Sun or Mars.
b) It is closer to Mars, than it is to either the Earth or the Sun.
c) It is half of the distance between the Sun and Mars.
d) It is closer to the Sun, than it is to either the Earth or Mars.
e) It is at the center of a triangle that has the Sun at one apex, the Earth at another apex, and Mars at the third apex.