in the world or of the world?

[Thompson-Tu]

Dear all,
I have a small question concerning the use of preposition. I'd like to know which preposition should be used in the sentence:
1. Heavy traffic is a big problem in many big cities in the world.
2. Heavy traffic is a big problem in many big cities of the world.
Please tell me which one is correct: in the world or of the world? Thanks.

[Mister Micawber]

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I think both are OK. Of is probably more usual, and is my choice.
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