Assertion `(A) : ` The presence of `CO_(2)` in the air accelerates corrosion.
Reason `(R): CO_(2)` is a poisonous gas.

Options

A. If both `(A)` and `(R)` are correct, and `(R)` is the correct explanation of `(A)`.
B. If both `(A)` and `(R)` are correct, but `(R)` is not the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R)` is incorrect.
D. If `(A)` is incorrect, `(R)` is correct.

Explanation

To analyze the given assertion and reason, let's break it down step by step:

### Step 1: Understanding the Assertion
The assertion states that the presence of \( CO_2 \) in the air accelerates corrosion.

- **Explanation**: Corrosion is a process where metals deteriorate due to chemical reactions, often involving moisture and gases in the environment. Carbon dioxide can indeed contribute to corrosion, particularly when it reacts with water to form carbonic acid (\( H_2CO_3 \)). This acid can lead to the corrosion of metals.

### Step 2: Evaluating the Reason
The reason provided is that \( CO_2 \) is a poisonous gas.

- **Explanation**: While \( CO_2 \) can be harmful in high concentrations, it is not classified as a poisonous gas in the same way that gases like carbon monoxide or hydrogen sulfide are. The assertion about \( CO_2 \) accelerating corrosion is true, but the reason given does not correctly explain why this occurs.

### Step 3: Conclusion
Based on the analysis:

- The assertion is **correct** because \( CO_2 \) does accelerate corrosion by forming carbonic acid.
- The reason is **incorrect** because it does not accurately explain the phenomenon of corrosion related to \( CO_2 \).

### Final Answer
Thus, the correct option is:
- **C**: Assertion is correct, Reason is incorrect.