Answer: C. Some of uranium's mass is converted into energy, so the smaller atoms have less mass.
Explanation:
From Einstein's mass-energy relation:
E = mc²
Mass and energy are equivalent. Mass can be converted into energy and energy into mass.
When Uranium atoms under go nuclear fission, smaller atoms are formed and huge amount of energy is released. This energy comes from the mass difference of the uranium nuclei and new nuclei formed. This mass converted into energy according to Einstein's equation.
They all have the same aphelion distances
to your question is 54 cm
The second one is best. Gravity is an attractive force that acts between all objects, whether they're in contact or not.
For a concave mirror, the radius of curvature is twice the focal length of the mirror:

where f, for a concave mirror, is taken to be positive.
Re-arranging the formula we get:

and since the radius of curvature of the mirror in the problem is 24 cm, the focal length is