某课外活动小组需要对两个定值电阻R₁、R₂和一只滑动变阻器R3的电阻进行测量，将这些元件连成如图所示的电路．闭合S₁，断开S₂，将滑动变阻器的阻值调到最大时，电流表的读数为0.2A，电压表的示数为2V；若将S₁、S₂都闭合，将滑动变阻器的阻值调到0时，电压表的示数为6V，电流表的示数为0.9A．求：

（1）R₁的阻值是多少？

（2）滑动变阻器的最大阻值R₃是多少？

（3）R₂的阻值是多少？

Clinical evidence began to________, suggesting that the new drugs had a wider range of useful activities

than had been predicted from experiments in animals.[    ]

A. operate  

B. strengthen

C. approve  

D. accumulate