$5ml$ of a gaseous hydrocarbon was exposed to $30ml$ of $O_2$.The resultant gas,on cooling is found to measure $25\;ml$ of which $10\;ml$ are obsorbed by NaOH and the remainder by pyrogallol.Determine molecular formula of hydrocarbons .All measurement are made at constant pressure and temperature.
$(a)\;CH_4\qquad(b)\;C_2H_4\qquad(c)\;C_2H_6\qquad(d)\;C_3H_8$