I've been observing a game of chess. Here are the first moves played:
1. Nf3 c6
2. c4 d6
3. Nd4 Qc7
4. e4 a5

What would you advice white to move next? Please only provide one move, without any commentary.
 Write your response with exactly 4 paragraphs, separated with the markdown divider: ***.
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