The total cost, in dollars, to order x units of a certain product is modeled by C(x)=5x²+320 . According to the model, for what size order is the cost per unit a minimum?

A) An order of 1 unit has a minimum cost per unit.

B) An order of 8 units has a minimum cost per unit.

C) An order of 80 units has a minimum cost per unit.

D) An order of 320 units has a minimum cost per unit.