After Joining Two Pieces of Picture Frame Molding Together, a Frame

Multiple Choice

After joining two pieces of picture frame molding together, a frame maker checks her work by making a diagonal measurement (see the illustration) . If the sides of the frame that form a right angle are a = 8 in.and b = 15 in., what measurement should the frame maker read on the yardstick?

A) 17 in.
B) $25$ in.
C) 21 in.
D) 23 in.
E) $15$ in.