Company Z reported depreciation expense of $15 million for the most recent year. Beginning-of-year gross PP&E and accumulated depreciation were $287 million and $77 million, respectively. If end-of year gross PP&E and accumulated depreciation were $300 million and $80 million, the estimated remaining useful life of PP&E is closest to:

a. 14.66 years

b. 10 years

c. 20.52 years

d. 12.54 years