A capital projects cash flows have an NPV of minus 40 when discounted at a rate of 15%; minus 60 when discounted at a rate of 20%; and plus 30 when discounted at a rate of 12%. The best approximation by linear interpolation of the projects percentage internal rate of return based on these data is:

a. 13.29% b. 15.27% c. 14.63% d. 16.12%