The correct response is:
It causes mixing of oxygenated (blood with oxygen in it) and deoxygenated blood (blood without oxygen in it), making it harder for the body to get the oxygen it needs.
In frogs, the heart has one ventricle, which leads to the mixing of oxygen-rich and oxygen-poor blood. This can reduce the efficiency of oxygen transport compared to organisms with separate chambers for oxygenated and deoxygenated blood.