A:13
B:-13
C:12
D:-12
阅读短文,判断正误。
My father has nice sungIasses. They’re five yuan. They‘re very cheap. My mother has a pretty scarf. I have an umbrella. It’s 50 yuan. It’s very expensive
如图是反射弧的组成示意图(虚线内为神经中枢),有关叙述正确的是( )
一定质量的木炭在氧气和氮气的混合气体的密闭容器中燃烧产生CO和CO2,且反应后测得混合气体中碳元素的质量分数为24%,则其中氮气的质量分数不可能为( )