Suppose that the average time before breakdown is normally distributed with a mean of 2.5 weeks and a standard deviation of .4 weeks. If breakdowns cost an average of $1,200 and preventive maintenance costs $800, what is the optimal maintenance interval?
A. Every .67 weeks
B. Every .33 weeks
C. Every 2.67 weeks
D. Every 3.25 weeks
E. Every 3.12 weeks