Which one of the following statements about the atmosphere is correct?

Options

A. The atmosphere has definite upper limits but gradually thins until it becomes imperceptible.
B. The atmosphere has no definite upper limits but gradually thins until it becomes imperceptible.
C. The atmosphere has definite upper limits but gradually thickens until it becomes imperceptible.
D. The atmosphere has no definite upper limits but gradually thickens until it becomes imperceptible.