标定硫酸标准溶液可用（）作基准物。
A、氢氧化钠；
B、氨水；
C、碳酸钠；
D、氯化钠。
参考答案：C
Tom , you ____ play with the knife. You_____ hurt you. [ ]
A. won't; can't
B. mustn't ; may
C. shouldn't ; must
D. can't ; shouldn't
按焊缝的结合形式不同可分为哪几类？
