Solve the Problem $c ( x ) = 2 x ^ { 2 } - 200 x + 87$

Solve the problem.
-Bob owns a watch repair shop. He has found that the cost of operating his shop is given by $c ( x ) = 2 x ^ { 2 } - 200 x + 87$ , where $x$ is the number of watches repaired. How many watches must he repair to have the lowest cost?

A) 20 watches
B) 43 watches
C) 87 watches
D) 50 watches