题目

阅读理解。National parks are large areas of public land.They give a safe home for local plants and animals.They help keep the air and water clean.They also give us the best trips.Today，there are nearly 7，000 national parks around the world.Forbes has listed 12 of the most beautiful ones in the world that will surprise you with their amazing landscapes，geographic wonders and colorful plants and animals.The Grand Canyon (大峡谷) National Park of the US is one of the Seven Natural Wonders of the World.It is best known for its size and depth.It is 446 kilometers long，up to 29 kilom海水是一个巨大的化学资源宝库，下列有关海水综合利用的说法正确的是( )A.海水中含有钾元素，只需经过物理变化就可以得到钾单质B.海水蒸发制海盐的过程中只发生了化学变化C.从海水中可以得到NaCl，电解熔融NaCl可制备Cl2D.利用潮汐发电是将化学能转化为电能