They worried about the purity of tap water.他们担心自来水是否纯净。The purity of the water is tested regularly.水的纯度定期检测。The students are asked to prove the purity of copper.要求学生们检验铜的纯度....
盐碱地和泥滩吸引了大量水鸟。The salt marshes and mud flats attract large numbers of waterfowl.许多水鸟在岛上栖息.A great number of water fowls dwell on the island.池塘上有一群水鸟.There is a flock of waterfowls on the pond....