Which of these fits gap 22?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
Pollution is one of the major ______ (21) ______ to the environment.  There are many types of pollution like air pollution, water pollution, soil pollution, noise pollution.  We must ____ (22) ____ towards reducing the pollution.  There are many ways to do this. For example, one can use vehicles and automobiles _____ (23) _____ on eco-friendly fuels.  One can also ____ (24) _____ the use of polythene bags.  One can even use natural manure in place of chemical fertilisers for the crops.