根据短文内容，选择正确答案。
D
    The students are having their chemistry (化学) class. Miss Liu is telling the children what water is like. After that, she asks her students, “What's water?”No one speaks for a few minutes. Miss Liu asks again, “Why don't you answer my question? Don't I tell you what water is like?”
    Then a boy puts up his hand and says, “Miss Liu, you tell us that water has no color and no smell. But where can we find such kind of water? The water in the river behind my house is always black and it has bad smell. "Most of the children agree with (同意) him.
    “I'm sorry, children.” says Miss Liu. “Our water is getting dirtier and dirtier. That's a problem.

如图，射线OA表示的方向是____．

看图回答下面问题

____克

为了研究影响蔬菜和水果水分蒸发快慢的因素，有 $\text{A}$ 、 $\text{B}$ 、 $\text{C}$ 、 $\text{D}$ 四组同学各自做了研究实验（实验材料是相同的胡萝卜），如下图所示，这四组实验中，研究方法不正确的是（    ）

0.5L 1mol/L FeCl₃溶液与0.2L1mol/L KCl溶液中的Cl^-的数目之比（      ）