3. A certain commodity has a purchase unit price of 40 yuan. If sold at 50 yuan each, 50 units can be sold; if the selling price increases by 1 yuan, the sales volume will decrease by 1 unit. To maximize profit, the optimal selling price for this commodity should be ( ) yuan per unit.
A. 50
B. 60
C. 70
D. 75
3. C.

Let the optimal selling price be $(50+x)$ yuan, profit $y=(50+x)(50-x)-40(50-x)=-(x-20)^{2}+900$.