Which of the following statements is incorrect?

Options

A. Pathogenic bacteria cause 90% of human diseases.
B. A large number of antiboitics are produced by actinomycets (e.g. Streplomyces), which are a class of fungi.
C. `N_(2)`. fixing bacteria pick up free `N_(2)` from soil atmosphere and convert it into nitrogenous compounds.
D. Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their suvival in extreme conditions.

Explanation

To determine which statement is incorrect, let's analyze each option step by step:

1. **Statement 1: Pathogenic bacteria cause 90% of human disease.**
- Pathogenic bacteria are indeed responsible for a significant number of diseases in humans. While the exact percentage may vary, it is generally accepted that a large proportion of infectious diseases are caused by bacteria. Therefore, this statement is **correct**.

2. **Statement 2: A large number of antibiotics are produced by actinomycetes or streptomycetes which are a class of fungi.**
- Actinomycetes and streptomycetes are known for their role in producing many antibiotics. However, they are not fungi; they are a group of bacteria. This makes this statement **incorrect**.

3. **Statement 3: N2 fixing bacteria pick up free N2 from soil atmosphere and convert it into nitrogenous compounds.**
- Nitrogen-fixing bacteria, such as Rhizobium, do indeed convert atmospheric nitrogen (N2) into usable forms like nitrates and ammonium compounds, which plants can absorb. This statement is **correct**.

4. **Statement 4: Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions.**
- Archaebacteria have unique cell wall structures, often containing ether bonds, which allow them to thrive in extreme environments. This statement is also **correct**.

Based on the analysis, the incorrect statement is **Statement 2**.

### Summary of the Solution:
- The incorrect statement is: "A large number of antibiotics are produced by actinomycetes or streptomycetes which are a class of fungi."