Which of the following predicate calculus statements is true?
a. ∀n in ℤ, ∀m in ℤ, n = m + 1
b. ∃n in ℤ, ∀m in ℤ, n + 1 = m
c. ∀n in ℤ, ∃m in ℤ, n + 1 = m
d. ∀n in Z, ∃m in ℤ, n + 1 = m