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 a frog's heart, the presence of a single ventricle leads to the mixing of oxygenated and deoxygenated blood, which can reduce the efficiency of oxygen delivery to the body compared to a system with separate ventricles, like that of humans.