(1)shall be construed so as not to include the following classes of purchases:-(a)a purchase of goods if the goods at the time of their purchase were the goods specified in Schedule I;(aa)a purchase of goods in respect of which no tax is payable under the proviso to sub-section (1) of section 4;(b)a purchase of goods from any person whose sale of such goods has been wholly exempted from payment of tax under section 49 or any notification issued thereunder;(c)a purchase of goods which is not liable to tax by virtue of the provisions of section 87, and
