Which of the following best describes the general trend of average binding energy per nucleon as the mass number increases from 1 to about 60?
(a) It increases roughly proportionally with mass number across the entire range.
(b) It remains almost constant over the entire range.
(c) It rises rapidly at first, then increases slowly, reaching a maximum near mass number 56.
(d) It rises at first, reaching a maximum near mass number 20, and then decreases gradually.