......... rays are used in killing bacteria in drinking water.

Options

A. IR
B. Red
C. UV
D. Green

Explanation

To solve the question "......... rays are used in killing bacteria in drinking water," we need to identify the type of rays that are effective in disinfecting water by killing bacteria.

### Step-by-Step Solution:

1. **Understand the Context**: The question asks about a specific type of rays used to kill bacteria in drinking water. This is a common topic in discussions about water purification methods.

2. **Consider the Options**: The options provided are:
- Infrared rays
- Red rays
- UV rays
- Green rays

3. **Evaluate Each Option**:
- **Infrared Rays**: These rays are primarily used for heating and do not have disinfecting properties.
- **Red Rays**: Similar to infrared, red rays do not have the capability to kill bacteria.
- **UV Rays**: Ultraviolet rays are known for their germicidal properties. They can effectively kill bacteria and viruses in water.
- **Green Rays**: These rays are not known for any disinfecting properties.

4. **Identify the Correct Answer**: Based on the evaluation, UV rays are the only option that is specifically known for killing bacteria in drinking water.

5. **Fill in the Blank**: Therefore, the blank in the question can be filled with "UV rays."

### Final Answer:
**UV rays are used in killing bacteria in drinking water.**

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