Which statement best explains why the atmosphere is considered an open system?

A There is no definite boundary separating it from outer space.

B Energy and matter can enter and leave.

C Its molecules can move freely, without resistance.

D It can accept an unlimited amount of energy.

The answer is B

An open system has its energy and matter leave and enter the system's boundaries unlike in a closed system where energy remains within the system.

The earth's atmosphere shares heat and water vapor with the hydrosphere (oceans, seas and lakes), energy form the sun, in the atmosphere and then partly radiated back to space