The first biocide used on a large scale was

Options

A. Boreaux mixture
B. DDT
C. Thurioside
D. Benzene haxachloride.

Answer

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the first biocide used on a large scale.

2. **Understand the Options**: The options provided are:
- Borax mixture
- DDT
- Thurocide
- Benzene hexachloride

3. **Recall Historical Context**: The use of biocides on a large scale became prominent during significant historical events, particularly during World War II.

4. **Analyze Each Option**:
- **Borax mixture**: While used for various purposes, it is not primarily known as a biocide on a large scale.
- **DDT**: Known for its widespread use during World War II as a pesticide, it was effective and became commercially significant.
- **Thurocide**: Not widely recognized or used on a large scale compared to DDT.
- **Benzene hexachloride**: Although used as an insecticide, it did not have the same level of large-scale application as DDT.

5. **Conclude with the Correct Answer**: Based on historical usage and effectiveness, the first biocide used on a large scale was DDT.

**Final Answer**: The first biocide used on a large scale was DDT.

---