In the context of the input sentence, determine the relationship between 'Diet Coke' and 'Coca-Cola'.
Options: industry, manufacturer, owned by, brand

However, 55 per cent said they considered it a substitute for Diet Coke and Coke Zero, suggesting that the new brand will cannibalise sales of low-calorie colas rather than generate incremental sales growth and restore volume growth to the brand, as Coca-Cola Amatil hopes.
brand