= Solution
The six pi <electrons> of B reorganize in a concerted <electrocyclic reaction>: the two terminal p orbitals form the new sigma bond, and the other two pi bonds shift to give the conjugated cyclohexadiene. Thermal six-electron closure is <disrotatory>, so the termini rotate in opposite senses. Applied to the terminal configurations drawn for B, this gives the trans methyl relationship. There is no need to invoke a radical chain or a sigmatropic transfer of a methyl group.
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