Select which of the following biconditionals is true. (Check all that apply.)
a. If n is even, then 7n + 4 is even; if 7n + 4 is even, then n is even.
b. If n is odd, then 7n + 4 is odd; if 7n + 4 is odd, then n is odd.
c. If n is even, then 7n + 4 is even; if 7n + 4 is odd, then n is odd.
d. If n is odd, then 7n + 4 is even; if 7n + 4 is even, then n is odd.