GPQA Diamond

What is the correct answer to this question?

Identify the EXO product of the following [4+2] cycloaddition reaction. 2,5-dimethylthiophene + Furan-2,5-dione + Heat ---> ?

Choices: (A) (3aR,4R,7S,7aS)-4,7-dimethyl-3a,4,7,7a-tetrahydro-4,7-epoxybenzo[c]thiophene-1,3-dione (B) (3aR,4S,7R,7aS)-4,7-dimethyl-3a,4,7,7a-tetrahydro-4,7-epoxybenzo[c]thiophene-1,3-dione (C) (3aR,4R,7S,7aS)-4,7-dimethyl-3a,4,7,7a-tetrahydro-4,7-epithioisobenzofuran-1,3-dione (D) (3aR,4S,7R,7aS)-4,7-dimethyl-3a,4,7,7a-tetrahydro-4,7-epithioisobenzofuran-1,3-dione

Format your final answer as "The answer is (X)" where X is A, B, C, or D.