According to the law of conservation of energy, which statement must be true?

D. The total energy of a system can increase only if energy enters the system

Explanation:

According to the law of Conservation of Energy, energy can neither be created nor destroyed but can be transformed or converted from one form to the other.

A system has the same energy until there is an addition of energy externally. When energy is added, it will lead to an increment in energy. So, when work is done to the system or the system heated, energy increases.