Why Is the Diels-Alder Reaction Called a Thermal [4+2] Cycloaddition $\pi$

Why is the Diels-Alder reaction called a thermal [4+2] cycloaddition?

A) Because the reaction is initiated by heat; the diene has four $\pi$ electrons and the dienophile has two $\pi$ electrons.
B) Because the reaction is initiated by light; the diene has four $\pi$ electrons and the dienophile has two $\pi$ electrons.
C) Because the reaction is initiated by heat; the dienophile has four $\pi$ electrons and the diene has two $\pi$ electrons.
D) Because the reaction is initiated by light; the dienophile has four $\pi$ electrons and the diene has two $\pi$ electrons.