If it is the abdomen and not the stomach area i would say its from the heat. if you've ever seen a dead animal on the side of the road over the course of a few days, they start to swell from the sun beating down on them all day.
Is there like options in this question?
Pollution plays a huge factor. With factories in demand, more and more substances are being released into Earth's atmosphere, resulting in higher temperatures.
The flow of energy is always one way. If we start at the sun and the sun’s energy moves to a plant, then the plant cannot transfer that energy back into the sun. The flow of energy is always controlled bc there is always a visible path it takes.