单项选择题
A．If they don’t involve any risks. B．If they produce predictable side effects. C．When the urgent need for them arises. D．When tests show that they are relatively safe.
A．Classmates, B．Colleagues. C．Boss and secretary. D．PR representative and client.
A．By greeting each other very politely. B．By exchanging their views on pubic affairs. C．By displaying their feelings and emotions. D．By asking each other some personal questions.