A Manufacturer of Lighting Fixtures Has Daily Production Costs Of C

Multiple Choice

A manufacturer of lighting fixtures has daily production costs of $C = 800 - 80 x + 0.25 x ^ { 2 }$ where C is the total cost (in dollars) and x is the number of units produced.How many fixtures should be produced each day to yield a minimum cost? (Round your answer to two decimal places. )
?

A) 160
B) 106.67
C) 320
D) 400
E) 800 ?