Question

1. what causes an explosion to occur?

objects at rest within a system have no momentum. an external force acts on the objects to move them apart, causing the explosion.
objects in motion within a system have momentum. an external force acts on the objects to move them apart.
objects at rest within a system have no momentum. an internal force acts on the objects to move them apart, causing the explosion.
objects in motion within a system have momentum. an internal force acts on the objects to move them apart, causing the explosion.

Explanation:

According to the law of conservation of momentum, in an explosion, the total initial momentum of the system (objects at rest within the system have no momentum initially). An internal force (such as the force from the explosion - generating mechanism like a chemical reaction in a bomb) acts on the objects to move them apart. External forces would affect the overall momentum of the system in a different way not characteristic of an explosion.

Answer:

Objects at rest within a system have no momentum. An internal force acts on the objects to move them apart, causing the explosion.