根据短文理解完成下列各题。
C
    Tom is a junior high student. He is good at some subjects but his favourite subject is chemistry. Yesterday he met some trouble when he had his chemistry class.
    At that moment, Tom was doing an experiment in class. He tried twenty times, but he couldn't succeed. So he decided to stop doing the experiment. He felt really unhappy.
    His teacher came up to him and asked why he was unhappy. Tom said, "All my work is in vain (白费力气). I fail twenty times and I get nothing."
    Tom's teacher touched Tom's head with a smile. He said to Tom, "Oh, you do a lot and you can get a lot from the mistakes. At least, you know that there are twenty ways you can't use to do this experiment."
    After hearing the teacher's words, Tom became excited. He kept doing the experiment, improved his methods, and he succeeded at last.

下列句中划线字不属词类活用的一项是

已知椭圆C： $\frac{x^2}{a^2}+\frac{y^2}{b^2}$ =1（a＞b＞0）的右焦点为F₂（1，0），点P（1， $\frac{\sqrt{2}}{2}$ ）在椭圆C上．

（Ⅰ）求椭圆C的方程；

（Ⅱ）过坐标原点O的两条直线EF，MN分别与椭圆C交于E，F，M，N四点，且直线OE，OM的斜率之积为﹣ $\frac{1}{2}$ ，求证：四边形EMFN的面积为定值．

一段水渠，已经修了 $\frac{4}{5}$ ，这里是把(    )看作单位“1"。

下列各组句子中，加线虚词的用法不同的一项是（　　）