The nuclear density of nuclides can be arranged.
The radius (r) of the nucleus is related to its mass number (A) as r^3 ∝ A.

a. Both A and R are true, and R is the correct explanation of A.
b. Both A and R are true, but R is not the correct explanation of A.
c. A is true, but R is false.
d. A is false, but R is true.