Thius question is from edexcel chemistry january 2014(mcq)

9 10 cm3
of a gaseous hydrocarbon was mixed with excess oxygen and ignited. The
gas volumes were measured at room temperature and pressure before and after
combustion and it was found that the total gas volume had contracted by 20 cm3
.
Given that combustion was complete, the formula of the hydrocarbon was
A C4
H4
B C4
H6
C C4
H8
D C4
H10
i dont know how to approach this question