Your dad has a low level of risk tolerance. You have explained time after time that his money will not grow unless he takes on the necessary level of risk. His current investment portfolio is very conservative. He only earned 4% on his portfolio in the past year. At this rate, if Dad currently has $75,000 invested in the stock market, how long will it take for his money to double?

a) 12 years

b) 16 years

c) 18 years

d) 9 years