Thin layer chromoatography $(TLC)$ is another type of adsorption chromatography, which involve sepration of substance of a mixture ovel a thin layer of an adsorbent coated on glass plate. A thin layer (about $0.2\,mm$ thick) of an adsorbent (silica gel) or (Alumina) in spread overa glass plate of suitable size. Hence Assertion $(A)$ is correct and Reason $(R)$ is correct explanation of $(A)$