According to the textbook, which of the following statements is (are) correct?

(x) The variable cost of 50 units equals the average variable cost of 50 units times 50.

(y) If the average fixed cost of producing 400 units is $42.25, then the average fixed cost of producing 1,750 units is less than $9.50 but more than $8.50.

(z) The marginal cost of the 12th unit equals the variable cost of 12 units minus the variable cost of 11 units

A. (x), (y) and (z)

B. (x) and (y) only

C. (x) and (z) only

D. (y) and (z) only

E. (z) only