First stage in respiration is

Options

A. Aerobic oxidation of pyruvic acid
B. Liberation of `CO_2`
C. Glycolysis
D. Electron transport system

Answer

To solve the question "First stage in respiration is," we need to analyze the options provided and understand the process of cellular respiration.

### Step-by-Step Solution:

1. **Understanding Cellular Respiration**: Cellular respiration is the process by which cells convert glucose into energy. It consists of several stages.

2. **Identifying the Stages**: The main stages of cellular respiration are:
- Glycolysis
- Pyruvate oxidation (intermediate stage)
- Krebs cycle (also known as the citric acid cycle)
- Electron transport system (ETS) or oxidative phosphorylation

3. **Determining the First Stage**: Among the stages listed, glycolysis is the very first step in the process of cellular respiration. It occurs in the cytoplasm of the cell and involves the breakdown of glucose into pyruvate.

4. **Analyzing the Options**:
- **Aerobic oxidation of pyruvic acid**: This occurs after glycolysis.
- **Liberation of CO2**: This occurs during the Krebs cycle.
- **Glycolysis**: This is the first step in the process.
- **Electron transport system**: This is the final stage of cellular respiration.

5. **Conclusion**: Since glycolysis is the first stage in the process of respiration, the correct answer is glycolysis.

### Final Answer:
The first stage in respiration is **glycolysis**.