Oxidation number of carbon in oxalate or oxalic acid

The same question, asked 2 times: 10 May 2023 S1*, 18 Apr 2026 S2. Every wording below, oldest first.

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Question 1
10 May 2023 S1
70 $\text{K}_2\text{C}_2\text{O}_4$
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Question 2
18 Apr 2026 S2
13 $What is the oxidation number of carbon in oxalic acid ?$
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