The greatest threat to the ozone layer is from
Directions: Read the following passage to answer the given question.
We are today faced with a great ecological problem for which we ourselves are largely responsible. Our planet, the Earth, the only home of man is bathed in a steady rain of the sun's rays. Most of these rays supply us heat and light, but a small percentage is deadly. These are the ultraviolet - B rays (or UV - B). If these were to get to the surface of the Earth directly, they would kill us off. Luckily, the Earth’s surface is shielded from these rays by the ozone layer; but sadly enough this ozone layer is being destroyed by man.
The ozone layer is formed from a kind of oxygen with three atoms (O3) instead of the normal two (O2). Ozone occurs naturally in the stratosphere, several kilometres above the Earth's surface. It absorbs the dangerous UV -B rays but allows the much needed and safe light to pass through. There, in the stratosphere, it is constantly being created by the sun's rays; thus it is a self-repairing shield. But man is now gradually destroying this vital shield with his industrial gases. The most lethal of these gases are the chlorofluorocarbons, or CFCs, which are used in making all kinds of foamed plastic products, propellants in spray cans, coolants in air conditioners and refrigerators and solvents to clean electronic equipment.
Down here, they are not lethal. But after they have leaked from air conditioners, plastic foam, cups, etc; they slowly drift up to the stratosphere where, bombarded by ultraviolet rays, they break up at last, releasing chlorine, the real ozone killer. Sadly, one chlorine molecule may remain active for a century, destroying thousands of ozone molecules.