配制250mL=0.050mol/L的AgNO3溶液,需称取含3.95%杂质的银()克。(MAg=108g/mol)
A、1.0
B、1.4
C、2.0
D、2.8
参考答案:B
(6分)一个耀眼的闪电过后8.4 s才传来雷声,雷声隆隆又持续6 s之久才停止,求发生的闪电的云层与观察者之间的距离。(声音在空气中的传播速度为340m/s)
The traffic is not busy now. We _____ arrive before dark. [ ]
A. should
B. mustn't
C. need
D. can't