What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: If Jennifer doesn't take a plane, then Charles is sad. It is not sunny. If Robert plays squash, then it is sunny. If Jennifer takes a plane, then Robert is playing squash.
Robert is not playing squash can be inferred via the modus tollens rule. If Jennifer takes a plane, then it is sunny can be inferred via the transitivity rule.