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.
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单项选择题
A．Classmates.B．Colleagues.C．Boss and secretary.D．PR representative and client.
A．Classmates.
B．Colleagues.
C．Boss and secretary.
D．PR representative and client.
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