Here's why:
* Abstract nouns refer to concepts, ideas, qualities, or states of being that cannot be touched or physically experienced.
* Common nouns refer to general things or people.
While the environment is made up of physical things (air, water, land), the word "environment" itself represents the *concept* of the surroundings or conditions in which something exists. It's not a tangible thing you can hold.
Here are some examples:
* Abstract: Love, happiness, freedom, justice, time, environment
* Common: Dog, book, table, city, person, tree