The photochemical cycloaddition of two molecules of c/s-2-butene gives a mixture of two products: A and B. The analogous photochemical cycloaddition of trans-2-butene also gives a mixture of two products: B and C. The photochemical reaction of a mixture of cis- and frarcs-2-butene gives a mixture of A, B, and C, along with a fourth product, D. Propose structures for all four compounds.