Two bodies with equal masses orbit the Sun in a circular orbit. If the radii of the two orbits are the same then following is true:

Question

a) The orbital period of the two bodies is the same because their masses are the same.
b) Their orbital period of the two cannot be the same because their orbital planes are
perpendicular to each other
c) Their orbital periods are the same because the radii is the same
d) none of the above.

I want to say the answer is c) because isn't period only dependent on radii?

The source of internal heat within the Earth is:
Geothermal energy
Radioactivity.
Earthquakes
Electromagnetic Energy

Is it radioactivity?

Answer 1

c) Kepler's third law. The square of the orbital period of a planet (time taken for a complete orbit) is directly proportional to the cube of the semi-major axis of its orbit (the average distance from the sun).

And, yes, radioactivity (from the uranium decay series).

Answer 2

Residual heat left over from the formation of the earth is also important. Much of the interior remains molten. Friction from tectonic plate motion and tidal forces (mainly due to the Moon) also generates heat.

See http://pubs.usgs.gov/gip/dynamic/unanswered.html