The age of an ancient artifact can be determined by the amount of radioactive carbon-14 remaining in it. If d0 is the original amount of carbon-14 and d is the amount remaining, then the artifact's age a (in years) is given by a = -8267 ln (d/d0). Find the age of an object if the amount d of carbon-14 that remains in the object is 87% of the original amount d0. (Round your answer to the nearest whole number.)
a. 2500 years
b. 2000 years
c. 1500 years
d. 1000 years