Early in the current year, Amazon Co. purchased the Rio Silver Mine at a cost of $30,000,000. The mine was estimated to contain 400,000 tons of ore and to have a residual value of $7,500,000 after mining operations are completed. During the year, 115,000 tons of ore were removed from the mine. At year-end, the book value of the mine is:

a. $23,531,250.
b. $6,468,750.
c. $22,500,000.
d. $30,000,000.