题目

It’s a popular belief that a fish’s memory lasts for only seven seconds. It may seem sad to think that they don’t remember what they’ve eaten or where they’ve been, and they don’t recognize you or any of their friends --- every moment in their life would be like seeing the world for the first time.        But don’t be so quick to feel sorry for them. A new study has found that fish have a much better memory than we used to think. In fact, certain species of fish can even remember events from as long as 12 days ago.        In the study, researchers from MacEwan 下列有关水的说法错误的是（       ） A．自然界中的河水、井水和湖水都属于混合物 B．实验室电解水时，正极端玻璃管内的气体是氢气 C．农业上合理的使用化肥和农药有利于水资源的保护 D．生活中可以通过煮沸降低水的硬度