Here's why:

* Complete combustion means that the alkene reacts with enough oxygen to fully oxidize all the carbon and hydrogen atoms.

* Alkenes are hydrocarbons with a carbon-carbon double bond. They have the general formula CₙH₂ₙ.

* Carbon dioxide (CO₂) is formed when carbon atoms in the alkene combine with oxygen.

* Water (H₂O) is formed when hydrogen atoms in the alkene combine with oxygen.

Here's a simplified chemical equation for the complete combustion of an alkene:

CₙH₂ₙ + (3n/2)O₂ → nCO₂ + nH₂O

Example:

The complete combustion of ethene (C₂H₄):

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O