Traffic restrictions have come into effect in Beijing      to improve the

capital’s air quality and traffic situation for the 2008 Olympics．

A．in order that

B．in an attempt

C．at an attempt

D．in a try

毒作用带描述，下述哪一项是错误的()

A．半数致死剂量与急性阈值的比值为急性毒作用带

B．急性阈剂量与慢性阈剂量的比值为慢性毒作用带

C．急性毒作用带值大，引起死亡的危险性大

D．慢性毒作用带值大，发生慢性中毒的危险性大