总铬测定时，用NO2-还原KMnO4，然后再用（）还原过量NO2-。
参考答案：CO（NH2）2
We were told that ______ when Marx was already _______, he started to learn Russian.
A．in 1840’s; in his fifties
B．in the 1840’s; his fifties
C．in the 1840’s; in his fifties
D．the 1840’s; in his fifties
_____,I think , and the problems could be settled .
A．If you double you efforts
B．So long as you keep up your spirts
C．Making greater efforts
D．A bit more efforts
