If it were aggregation then the aggregated object would have a subsidiary lifecycle to the container. I.e. if the container goes away, so does the contained object. From the Wikipedia page for Composition
Composition is a kind of association where the composite object has sole responsibility for the disposition of the component parts. The relationship between the composite and the component is a strong “has a” relationship, as the composite object takes ownership of the component. This means the composite is responsible for the creation and destruction of the component parts.