Which of the Following Statements About [2+2] Cycloaddition Reactions Between $\sigma$

Which of the following statements about [2+2] cycloaddition reactions between two alkenes is true?

A) The reaction is initiated by heat.
B) The reaction is initiated by light.
C) The product is a cyclopentane derivative.
D) Each reactant contains two $\sigma$ electrons that participate in the formation of new bonds.