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

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

A) Because the reaction is initiated by light; the dienophile has fourp electrons and the diene has twop electrons.
B) Because the reaction is initiated by heat; the diene has fourp electrons and the dienophile has twop electrons.
C) Because the reaction is initiated by light; the diene has fourp electrons and the dienophile has twop electrons.
D) Because the reaction is initiated by heat; the dienophile has fourp electrons and the diene has twop electrons.