<P> The tetrahedron is unique among the uniform polyhedra in possessing no parallel faces . </P> <P> A corollary of the usual law of sines is that in a tetrahedron with vertices O, A, B, C, we have </P> <Dl> <Dd> sin ⁡ ∠ O A B ⋅ sin ⁡ ∠ O B C ⋅ sin ⁡ ∠ O C A = sin ⁡ ∠ O A C ⋅ sin ⁡ ∠ O C B ⋅ sin ⁡ ∠ O B A . (\ displaystyle \ sin \ angle OAB \ cdot \ sin \ angle OBC \ cdot \ sin \ angle OCA = \ sin \ angle OAC \ cdot \ sin \ angle OCB \ cdot \ sin \ angle OBA. \,) </Dd> </Dl> <Dd> sin ⁡ ∠ O A B ⋅ sin ⁡ ∠ O B C ⋅ sin ⁡ ∠ O C A = sin ⁡ ∠ O A C ⋅ sin ⁡ ∠ O C B ⋅ sin ⁡ ∠ O B A . (\ displaystyle \ sin \ angle OAB \ cdot \ sin \ angle OBC \ cdot \ sin \ angle OCA = \ sin \ angle OAC \ cdot \ sin \ angle OCB \ cdot \ sin \ angle OBA. \,) </Dd>

Which of the following nets could be folded on the edges to form a tetrahedron