Why is the Law of Conservation of mass a law and not a theory?

Iknow #2 is d. i just need to know why

Select the word from the list that best fits the definition the nuclear family into which a person is born or adopted.

In a heat of an engine, if 700 j enters the system, and the piston does 400 j of work what is the final internal (thermal) energy of the system if the initial energy is 1,500 j