Which statement is true?
when an apple falls from a tree, the forces present are the gravitational forces
of the apple and the earth.
when an apple falls from a tree, the forces present are the gravitational force of
the earth and the moon.
when an apple falls from a tree, the only force present is the gravitational force
of the earth on the apple.
when an apple falls from a tree, the only force present is the gravitational force
of the apple on the earth.

The correct option is 'When an apple falls from a tree, the only force present is the gravitational force  of the earth on the apple.'

Explanation:

Gravity can be described as a force which allows two objects to be attracted towards each other. The gravitational force allows the planets to move around the Sun. It is due to the gravitational pull of the moon that tides are caused in an ocean.

The gravitational force of the Earth tends to pull an object in the downward direction. As a result, everything that we throw up falls down because of the gravitational pull of the Earth. Hence, an apple falling will experience a gravitational force from the Earth.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.