Chemical reactions involve changes - changes of reactants into products. Basic chemical reactions can be grouped into categories based on the types of changes that are occuring during the reaction. There are five basic categories - synthesis, decomposition, combustion, single replacement, and double replacement. If you know the type of reaction that reactants will involve themselves in, then you can predict the products that will be produced as the result of that reaction.
 

A combustion reaction involves the reaction of a substance with oxygen gas. The most common combustible substance is a hydrocarbon - substance containing the elements carbon and hydrogen. If the substance undergoing combustion is a hydrocarbon, then the products of the combustion are carbon dioxide (CO₂) and water vapor (H₂O). And that's a fact that's worth committing to memory ... and a fact that will get you the answer to this question rather quickly.