Dry ice is

Options

A. solid `CO_2`
B. solid `CO`
C. solid `SO_2`
D. solid `He`

Explanation

To determine what dry ice is, we need to analyze the options provided and identify the correct one.

### Step-by-Step Solution:

1. **Understanding Dry Ice**:
- Dry ice is a common term used for the solid form of carbon dioxide (CO2). It is used primarily as a refrigerant and in various industrial applications.

2. **Analyzing the Options**:
- **Option 1: Solid CO2**: This option refers to solid carbon dioxide, which is indeed what dry ice is.
- **Option 2: Solid CO**: This option refers to solid carbon monoxide, which is not dry ice.
- **Option 3: Solid SO2**: This option refers to solid sulfur dioxide, which is also not dry ice.
- **Option 4: Solid Helium**: This option refers to solid helium, which is not related to dry ice.

3. **Identifying the Correct Answer**:
- Since dry ice is defined as solid carbon dioxide, the correct answer is **Option 1: Solid CO2**.

### Final Answer:
Dry ice is **Solid CO2**.

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