1976 AHSME Problem 14

The measures of the interior angles of a convex polygon are in arithmetic progression. If the smallest angle is 100^\circ, and the largest is 140^\circ, then the number of sides the polygon has is

\textbf{(A) }6\qquad \textbf{(B) }8\qquad \textbf{(C) }10\qquad \textbf{(D) }11\qquad \textbf{(E) }12

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.