A study of peach trees found that the average number of peaches per tree was 725. The standard deviation of the population is 105 peaches per tree. A scientist wishes to find the 90% confidence interval for the mean number of peaches per tree. How many trees does she need to sample to obtain an average accurate to within 18 peaches per tree?
A. 35
B. 56
C. 72
D. 93