While adding the first n terms of an arithmetic progression, a student pairs the first term with the last term, the second term with the second–last term, and so on.
Why does every such pair have the same sum?
a) Because a+l equals the common difference of the progression.
b) Because terms equally distant from the ends always add up to the constant value a+l.
c) Because terms next to each other always add up to the same value since the common difference is constant.
d) Because a+l is twice the common difference of the progression.