I've been observing a game of chess. Here are the first moves played:
1. Nf3 Na6
2. Nh4 b5
3. f3 Nh6
4. Na3 d5

What would you advice white to move next? Please only provide one move, without any commentary.
 Write your response with exactly 5 paragraphs, separated with the markdown divider: ***.
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