Martin invested $1,000 six years ago and expected to have $1,500 today. He has not added or withdrawn any money from this account since his initial investment. All interest was reinvested in the account. As it turns out, Martin only has $1,520 in his account today. Which one of the following must be true? Martin earned simple interest rather than compound interest Martin earned a lower interest rate than he expected. Martin did not earn any interest on interest as he expected. Martin ignored the Rule of 72 which caused his account to decrease in value. Martin earned a higher interest rate than he expected.