Passage Summary: In this town, most shops have birds, and many of those have fish too. If a shop has fish but no birds, it definitely has gerbils. Also, none of the independent shops carry gerbils.

Reasoning: Most pet stores sell exotic birds; most of those also sell tropical fish. Stores with fish but no birds must sell gerbils. No independent stores sell gerbils.

Analysis: This is a Must Be True question involving formal logic and overlapping groups. We know that 'Fish + No Birds = Gerbils.' We also know 'Independent = No Gerbils.' By taking the contrapositive of the first rule, we see that 'No Gerbils = No Fish OR Birds.' Combining these, if an independent store sells tropical fish, it cannot be in the 'No Birds' category because that would require it to sell gerbils. Therefore, any independent store selling tropical fish must also sell exotic birds. Look for a synthesis of these conditional chains.