The sum of two positive integers, a and b, is at least 30. the difference of the two integers is at least 10. if b is the greater integer, which system of inequalities could represent the values of a and b?
a. a + b ≥ 30 b ≥ a + 10
b. a + b ≥ 30 b ≤ a – 10
c. a + b ≤ 30 b ≥ a + 10
d. a + b ≤ 30 b ≤ a – 10