The oxygen dissociation curve is

Options

A. parabola
B. slope
C. sigmoid
D. stright line.

Explanation

### Step-by-Step Solution:

1. **Understanding the Oxygen Dissociation Curve**: The oxygen dissociation curve illustrates the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen. It helps to understand how readily hemoglobin acquires and releases oxygen.

2. **Graph Representation**: To visualize this relationship, we plot a graph where the x-axis represents the partial pressure of oxygen (measured in mm Hg) and the y-axis represents the percentage saturation of hemoglobin with oxygen.

3. **Shape of the Curve**: As we plot the data points, we observe that the curve does not form a straight line. Instead, it exhibits a characteristic S-shape, which indicates that hemoglobin's affinity for oxygen changes at different levels of pO2.

4. **Sigmoid Nature**: The S-shaped curve is referred to as a sigmoid curve. This shape indicates that at low pO2 levels, hemoglobin binds oxygen slowly, but as pO2 increases, hemoglobin's affinity for oxygen increases rapidly until it reaches a plateau at high pO2 levels.

5. **Conclusion**: Based on the analysis of the graph, we conclude that the oxygen dissociation curve is a sigmoid curve. Therefore, the correct answer is option C (sigmoid).