我认为选A.
大气和地球上的土壤,湖水、河水、海水一样,也是地球的一部分.
可以这么写比较容易理
As are its soils and the water of its lakes,rivers and oceans,the atmosphere is as much a part of the earth.
前半句的意思是soils and the water of its lakes,rivers and oceans is parts of the earth.
as much的意思是“也”,相当于一个副词.its soils 是复数,根据就近原则所以用are.