Identify Grammatically Correct Sentences

Each of the questions consists of a certain number of sentences. Some sentences are grammatically incorrect or inappropriate. Select the option that indicates the grammatically correct and appropriate sentence(s).

1. Large reductions in the ozone layer, which sits about 15-30 km above the Earth, take place each winter over the Polar regions, especially the Antarctic, as low temperatures allow the formation of stratospheric clouds that assist chemical reactions breaking down ozone.
2. Industrial chemicals containing chlorine and bromine have been blamed for thinning the layer because they attack the ozone molecules, making them to break apart.
3. Many an offending chemicals have now been banned.
4. It will still take several decades before these substances have disappeared from the atmosphere.