Solve the Problem $x$ Thousand Automobiles Is Given by the Function $C ( x ) = 5 x ^ { 2 } - 30 x + 81$

Multiple Choice

Solve the problem.
-The cost in millions of dollars for a company to manufacture $x$ thousand automobiles is given by the function $C ( x ) = 5 x ^ { 2 } - 30 x + 81$ . Find the number of automobiles that must be produced to minimize the cost.

A) 3 thousand automobiles
B) 6 thousand automobiles
C) 36 thousand automobiles
D) 15 thousand automobiles