Miles invested $$5,000$ ten years ago and expected to have $$10,000$ today. He has neither added nor withdrawn any money since his initial investment. All interest was reinvested and compounded annually. As it turns out, he only has $$8,400$ in his account today. Which one of the following statements must be true?

He earned simple interest rather than compound interest.

He earned a lower interest rate than he expected.

He did not earn any interest on interest.

He ignored the Rule of $72,$ which caused his account to decrease in value.

The future value interest factor turned out to be higher than he expected.