Phillippe invested $1,000 ten years ago and expected to have $1,800 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 He has not added or withdrawn any money from this account since his initial investment.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, he only has $1,680 in his account today. Which one of the following must be true?

He earned simple interest rather than compound interest.

He did not earn any interest on interest as he expected.

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

He earned a lower interest rate than he expected.

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