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: For all x_18, if x_18 were happy, then x_18 is climbing a mountain. For all x_18, x_18 is sad if and only if x_18 will climb a mountain. For all x_18, x_18 is sad.
For all x_18, if x_18 were sad, then x_18 is climbing a mountain can be inferred via the universal biconditional elimination rule. For all x_18, if x_18 climbs a mountain, then x_18 is sad can be inferred via the universal biconditional elimination rule.