(a) Air is a compressible gas and a light element that can overcome frictional forces. The availability of air everywhere also supports the fact that air is used to inflate tyres.

(b) Steel is used to make railway lines because it is very flexible and has the ability to expand. It is seen that during summers, metals expand. Steel is used because when it expands it does not change its shape. Steel does not get rust which makes it suitable for this purpose.