There are four oceans on the earth Ocean 为什么要加s

There are four oceans on the earth Ocean 为什么要加s There are five oceans on the earth. there are air and water on the Earth.为什么要用are The oceans are very important to all animals on the earth.(改 There are trees on the earth. There are four ____________ on the floor . There are a lot of trees on the both of bank 中bank是否要加s?为什么? How many rulers are there on the desk?Four这个句子对还是错,要讲明为什么的哦? what are there on earth There are many animals and p（ ）on the earth Today there are 6 billion people on the earth. how many continents are there on the earth