5. what causes the plates to be pulled apart?

Brief Explanations:

Convection currents in the mantle drive the movement of tectonic plates. These currents are caused by the heat from the Earth's interior. As the hot, less - dense material rises and the cooler, denser material sinks, it exerts a force on the overlying tectonic plates, pulling them apart at divergent plate boundaries.

Answer:

Convection currents in the mantle.