我国的水电站主要集中在水力资源丰富的____ 江、____ 、珠江、____ 江，松花江____ 游．

下列学生习作中语句使用得体的一项是（    ）

阅读下列短文，从每小题的三个选项中选出最佳选项。B
    Water comes from the sea, the rain and the tap. Animals, plants and human beings all need water. It is important for our daily (日常的）life. We use water to wash clothes and vegetables. We use water to cook food, make drinks, wash our face and so on. Firemen use water to put out fires. Fanners use water to grow crops. Water is useful. We mustn't waste (浪费）water. We must save water. Turn off the tap when you are brushing your teeth. Don't take a shower too long. Use the same water many times.

根据体质特征的不同，把现代人类分成四个种族，分别是____、____、____、____．

买一支钢笔需要a元，买一本笔记本需要b元，那么买m支钢笔和n本笔记本需要____元．