Fill in the blank.
________ acid on reaction with `H_2SO_4` gives only carbon monoxide gas.

Explanation

To solve the question, we need to identify the acid that, when reacted with sulfuric acid (H₂SO₄), produces only carbon monoxide gas (CO).

### Step-by-Step Solution:

1. **Identify the type of acid**: The question specifies that we are looking for an acid that reacts with H₂SO₄ to produce carbon monoxide. The simplest type of acid that fits this description is a carboxylic acid.

2. **Consider the simplest carboxylic acid**: The simplest carboxylic acid is formic acid, which is also known as methanoic acid. Its chemical formula is HCOOH.

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