Which of these statements are correct regarding the atmosphere of the earth:1. It is enveloping the earth because of gravity.2. Its density decreases with increasing height from the surface of the earth.3. Its characteristic composition is same throughout its height.4. Amount of pressure it exerts on earth's surface at sea level is called one atm sphere.

• A. 1 and 4
• B. 2 and 4
• C. 2, 3 and 4
• D. 1, 2 and 4