According to the rule of 72, t=72/r which statement is true?

a. an investment of $3,100 will double in 12 years at a compound interest rate of 5%.

b. an investment of $9,000 will double in 10 years at a compound interest rate of 7%.

c. an investment of $4,500 will double in 8 years at a compound interest rate of 9%.

d. an investment of $3,000 will double in 4 years at a compound interest rate of 12%.