To prepare a blood smear slide a student placed a drop of blood on a glass slide and added a drop of distilled water to dilute the blood. He then added stain (a coloured chemical used to see the cells) to it before placing the coverslip over it. When viewed, under the microscope he could not see the blood cells. Which of the following best explains this?

Options

A. The blood cells had burst.
B. The blood had dried before the water was added.
C. The drop of water diluted the blood too much and cells dispersed too far away from each other.
D. The water caused the cells to shrink.

Explanation

To solve the question regarding why the student could not see the blood cells under the microscope after preparing the blood smear slide, we can break down the process step by step:

### Step-by-Step Solution:

1. **Understanding the Blood Smear Preparation**:
- The student placed a drop of blood on a glass slide and added distilled water to it. This is a common practice to dilute the blood for better visualization of cells.

2. **Role of Stain**:
- After diluting the blood with distilled water, the student added a stain. Stains are used to enhance the visibility of cells under a microscope by coloring them.

3. **Observation Under the Microscope**:
- When the student viewed the slide under the microscope, he could not see the blood cells. This indicates that something went wrong during the preparation.

4. **Analyzing the Effect of Distilled Water**:
- Adding distilled water to the blood can lead to a process called endosmosis. In this process, water enters the blood cells because the concentration of solutes inside the cells is higher than in the distilled water.

5. **Consequences of Endosmosis**:
- As water enters the cells, they may swell and eventually burst due to the excessive influx of water. This bursting of cells would result in the absence of visible blood cells on the slide.

6. **Conclusion**:
- The best explanation for why the student could not see the blood cells is that the blood cells had burst due to the osmotic pressure created by the addition of distilled water.

### Final Answer:
The blood cells had burst due to the addition of distilled water, which caused endosmosis.

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