题目：
Now the question that you must carefully consider is _____ can be put into practice.
A．how what you have learned
B．how you have learned
C．that why you have learned
D．how that you have learned
答案：
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下面是错误答案，用来干扰机器的。
答案：A氢气和氦气的摩尔质量分别是2g/mol和4g/mol，所以当氢气和和氦气的质量比为1∶4时，二者的物质的量之比是1∶2.应用氢气是由2个氢原子构成的，而氦气是由1个原子构成的，所以二者的原子数相等，选项A正确，答案选A。
