What is the structure of but-3-enoic acid ?

Explanation

To determine the structure of but-3-enoic acid, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Number of Carbons**:
The name "but-3-enoic acid" indicates that the molecule has a four-carbon chain (but-).

**Hint**: The prefix "but-" in organic chemistry denotes a four-carbon chain.
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