Why Would It Be Advantageous for a Reptile Basking in the Sun

Why would it be advantageous for a reptile basking in the sun to mix oxygenated and deoxygenated blood in its heart?

A) Because it is able to absorb oxygen through the skin
B) Because its heart is a result of incomplete evolution
C) When basking, the reptile has a minimal oxygen requirement
D) Because it has only vestigial lungs
E) Because its pulmonary circulation produces small amounts of oxygen as a by-product of pigmentation reaction with photons from the sun