What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all x, x will take a plane if and only if x will play squash. For all x, x will play squash if and only if x will take a plane.
For all x, if x takes a plane, then x is playing squash. For all x, if x plays squash, then x is taking a plane. For all x, if x plays squash, then x is taking a plane. For all x, if x takes a plane, then x is playing squash.