Examine how nearly all energy for life comes directly or indirectly from the sun given that heterotrophs get their energy from the food they eat

Well the food that heterotrophs eat are either the autotrophs who photosynthesize, or other heterotrophs. When they eat the autotrophs the energy stored in the plant matter gets transferred to them. If another animal was to eat the initial consumer of the autotrophs then they would be getting the energy stored in that animal that was obtained by eating the autotrophs who had the initial energy.

The answer is a because - is an direction