Suppose that the duration of trouble-free operation of a new vacuum cleaner is normally distributed with a mean of 530 days and a standard deviation of 100 days. The research department randomly selects 25 customers who bought these vacuum cleaners and records the duration (in days) of trouble-free operation. Which of the following is TRUE for the sample average of number of trouble-free operation days ()?

A. P( x <500) = P(Z < -1.5)

B. P(x <500) = 0.9332

C. P(x <520) = P(Z < 2.5)

D. P( x <520) = 0.4938