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 y_1, if y_1 were a taxi driver, then y_1 is climbing a mountain. For all y_1, y_1 will not climb a mountain. For all y_1, if y_1 were a taxi driver, then y_1 is climbing a mountain. For all y_1, if y_1 climbs a mountain, then y_1 is a taxi driver.
For all y_1, y_1 is not a taxi driver can be inferred via the universal modus tollens rule. For all y_1, y_1 is not a taxi driver can be inferred via the universal modus tollens rule. For all y_1, y_1 is a taxi driver if and only if y_1 will climb a mountain can be inferred via the universal biconditional introduction rule. For all y_1, y_1 is a taxi driver if and only if y_1 will climb a mountain can be inferred via the universal biconditional introduction rule. For all y_1, y_1 will climb a mountain if and only if y_1 is a taxi driver can be inferred via the universal biconditional introduction rule. For all y_1, y_1 will climb a mountain if and only if y_1 is a taxi driver can be inferred via the universal biconditional introduction rule.