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Somewhere in Pennsylvania

This organization is located in one of the largest cities in the Commonwealth of Pennsylvania.

Based on the logo’s negative space images, what are the starting letters of the two key words revealing the nature of the organization?

Explanations

The negative space in this logo depicts a gorilla and a lion on two sides of the tree, and a pair of fish below them. This is the logo of the Pittsburgh Zoo & PPG Aquarium. The key words, revealing the nature of the organization, are Zoo and Aquarium – or the Z & A respectively.

The Pittsburgh Zoo, opened on June 14, 1898, is one of only six major zoo and aquarium combinations in the United States. [Wikipedia]

Check A & B
Check X & Y
Check Y & Z
Check Z & A