Name the drying agent:
which is used in drying hydrogen chloride gas.

Explanation

### Step-by-Step Solution

1. **Understand the Question**: The question asks for the name of the drying agent used to dry hydrogen chloride gas (HCl).

2. **Identify the Chemical Formula**: Recognize that hydrogen chloride gas is represented by the formula HCl.

3. **Know the Purpose of a Drying Agent**: A drying agent is a substance used to remove moisture from gases or liquids. In the case of HCl, we need a drying agent that does not react with it.

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