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Chemistry help - urgent!

100 cm 3 of hydrogen is mixed with 25 cm 3 of oxygen at a temperature of 150°C.
The gases react as shown in the equation below.

2H2 (g) + O2 (g) --> 2H2O(g)
The total volume of gas present at the end of the reaction is
***A ** 50 cm 3
***B ** 100 cm 3
***C ** 125 cm 3
***D ** 150 cm 3

I keep getting 125cm3, I worked out the excess volume to be 75, then added it to the product volume which is 50, but the answer is 100.. can anyone please help me I have an exam tomorrow on this!! thank you!!!