I'd go with A, if I were you.
Animals can be classified into two main groups: vertebrates and invertebrates. The main difference between vertebrates and invertebrates is that invertebrates, like insects and flatworms, do not have a backbone or a spinal column. Examples of vertebrates include humans, birds, and snakes.
the part of the heart takes blood from the veins and pumps it into ventricle is called atria
