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 David takes a plane, then Jessica is sad. Thomas is writing a letter if and only if Jessica is sad. David is taking a plane or Thomas is writing a letter.
If Thomas writes a letter, then Jessica is sad can be inferred via the biconditional elimination rule. If Jessica were sad, then Thomas is writing a letter can be inferred via the biconditional elimination rule.