Work on books$|$ Problem

Consider some books at rest on a shelf in your room.$$ Which of the following statements are true?

Group of answer choices

Zero work is being done on the books to hold them up$.$

Positive work is being done on the books to push against gravity.

Negative work is being done on the books since they make your shelf sag slightly.

The mechanical energy of the books $+$ Earth system is not changing.

The mechanical energy of the books $+$ Earth system is increasing.

The mechanical energy of the books $+$ Earth system is decreasing.