According to your text, mashup apps are designed based on three "tiers". The warehouse tier is where information is stored and retrieved from a database or file system.

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Quizplus · Accepted Answer

Mashup apps are typically described as having three layers or tiers: the presentation tier (user interface), the logic tier (where data is manipulated and managed), and the data tier (where data is stored and retrieved), not a "warehouse tier."

According to Your Text, Mashup Apps Are Designed Based on Three