若向量 $\stackrel{\rightharpoonup }{AB}=\left(1,2\right)$ ， $\stackrel{\rightharpoonup }{BC}=\left(-4,2\right)$ ，则 $\left|\stackrel{\rightharpoonup }{AC}\right|=$ （    ）

阅读理解
    Different weather makes people feel different．It influences health，intelligence and feelings．
    In August，it is very hot and wet in the south the US．People there have heart trouble and other kinds of health problems during this month．In the Northeast and the Middle West，it is very hot in the summer and very cold in the winter．People in these states have more heart trouble after the weather changes in February or March．
    The weather can also influence intelligence ． For example，in a 1983report by scientists，IQ(智商)of a group of students was very high when a very strong wind came, but after the strong wind,
    Their IQ was 10% lower than before. The wind can help people have higher intelligence. However,very hot weather can make it lower. Students in many schools of the US often get worse in exams(考试)in the hot months of the year(July and August).
    Weather also has a strong influence on people's feelings．Winter may be a bad time for thin people．They usually feel cold during these months．They might feel unhappy during cold weather．But fat people may have a hard time in hot summer．At about 18℃people become stronger．Low air pressure may make people forgetful．People leave more bags on buses and in shops on low-pressure days．
    Are you feeling sad，tired，forgetful，or unhappy today？It may be the weather's problem．

计算2x³÷ $\frac{1}{x}$ 的结果是（   ）

如图,直线l₁∥l₂,CD⊥AB于点D,∠1=50°,则∠BCD的度数为(   )

如图所示的转化关系中，“﹣”表示两种物质之间可以相互反应，但能直接转化，已知A为白色固体，B、D是两种人体呼吸有关的常见气体，则A的化学式为____；C和D发生反应的化学方程式____。