How many molecules of water are needed by a green plant to reduce 6 molecules of `CO_2` ?

Options

A. 6
B. 12
C. 24
D. One only

Answer

To determine how many molecules of water are needed by a green plant to reduce 6 molecules of carbon dioxide (CO₂), we can analyze the process of photosynthesis.

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
- Photosynthesis is the process by which green plants convert carbon dioxide and water into glucose and oxygen using sunlight.
- The general equation for photosynthesis is:
\[ 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \]
- In this equation, 6 molecules of carbon dioxide react with 6 molecules of water to produce 1 molecule of glucose and 6 molecules of oxygen.

2. **Identifying the Relationship**:
- From the equation, we can see that for every 6 molecules of CO₂, 6 molecules of H₂O are needed to produce glucose and oxygen.
- However, we need to consider the stoichiometry of the reaction.

3. **Calculating Water Requirement**:
- The balanced equation shows that to produce glucose from 6 molecules of CO₂, 12 molecules of water (H₂O) are actually required.
- This is because water is oxidized in the process, contributing to the formation of glucose and releasing oxygen.

4. **Final Answer**:
- Therefore, to reduce 6 molecules of CO₂, a green plant needs **12 molecules of water**.

### Conclusion:
The answer to the question is **B - 12**.