Assertion (A) : a, b, c are in A.P. if and only if 2b = a + c
Reason (R) : The sum of first n odd natural numbers is n²

a. Both assertion and reason are correct and reason is the correct explanation
b. Both assertion and reason are correct but reason is not the correct explanation for assertion
c. Assertion is correct but reason is incorrect
d. Assertion is incorrect but reason is correct