The current availability of `CO_(2)` levels is limiting to the

Options

A. `C_(3)` plants
B. `C_(4)` plants
C. CAM plants
D. Both A and B

Explanation

To answer the question regarding the limiting effect of current CO2 levels, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Photosynthesis**: Photosynthesis is the process by which plants convert light energy into chemical energy, using carbon dioxide (CO2) and water (H2O) to produce glucose and oxygen (O2).

2. **Role of CO2**: Carbon dioxide is a crucial reactant in the photosynthesis process. The concentration of CO2 directly affects the rate of photosynthesis. If CO2 levels are low, the rate of photosynthesis decreases.

3. **C3 Plants and Photorespiration**: C3 plants are a type of plant that utilize the Calvin cycle for carbon fixation. However, when CO2 levels are low, these plants can undergo a process called photorespiration. This occurs when the enzyme RuBisCO, which normally fixes CO2, instead binds to oxygen (O2), leading to the production of a 2-carbon compound instead of glucose.

4. **Consequences of Photorespiration**: Photorespiration is considered an energy-wasting process because it consumes energy and releases CO2 without producing glucose. This process is more likely to occur when CO2 levels are low and O2 levels are high.

5. **Conclusion**: Therefore, the current availability of CO2 levels is limiting to C3 plants because they are more susceptible to photorespiration under low CO2 conditions.

### Final Answer:
The current availability of CO2 levels is limiting to C3 plants.

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