8. A convex decagon has 10 interior angles. What is the maximum number of them that can be right angles?
(A) 5
(B) 3
(C) 4
(D) 2
(E) 6
(8) The correct answer is $(B)$.

For each right interior angle, the exterior angle (which is its supplement) is also right. Since in any convex polygon the sum of the exterior angles is equal to a full angle, it follows that there cannot be more than 3 right angles (unless it is a rectangle, which is to be excluded in the current case). On the other hand, it is not difficult to construct explicit convex polygons with 10 sides having exactly 3 right angles.

Question proposed by Carlo Cassola.