You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

[Q]: Category: MILITARY TRAINING BASES 
Clue: Service branch with a San Diego recruit depot & a Yuma air station
[A]: marines


[Q]: Category: WORLD HISTORY 
Clue: He completed vols. 2 & 3 of Karl Marx's "Das Kapital"
[A]: friedrich engels


[Q]: Category: "E" CHANNEL 
Clue: In 1974 Spokane's Cannon Island was the site of this, which featured an environmental theme
[A]:
an expo