The effort involved in lying produces measurable physiological reactions such as a speedup of the heartbeat. Since lying is accompanied by physiological reactions, lie-detector tests that can detect these reactions are a sure way of determining when someone is lying. Which one of the following statements, if true, most seriously weakens the argument? ['Lie-detector tests can measure only some of the physiological reactions that occur when someone is lying.', 'When employers use lie-detector tests as part of their preemployment screening, some candidates tested are highly motivated to lie.', 'People are often unaware that they are having physiological reactions of the sort measured by lie-detector tests.', 'For people who are not lying, the tension of taking a lie-detector test can produce physiological reactions identical to the ones that accompany the act of lying.']