Mother tongue is very helpful for students.
Easily comprehend means easily understand.
Cooperation is the action or process of working together to the same end.
"they worked in close cooperation with the British Tourist Authority"
Major forms of pollution include air pollution, light pollution, noise pollution, plastic pollution, soil contamination, radioactive contamination, thermal pollution,and water pollution.
Until indicates when something will happen, begin, or end.
Till means the same thing as until.
Till is not an abbreviation of until—it's actually older than until—and it should not be written with an apostrophe.
A bar graph is a graph that presents categorical data with rectangular bars with heights or lengths proportional to the values that they represent.