Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty (b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
Why is potato A necessary for this experiment?

Explanation

### Step-by-Step Solution:

1. **Understanding the Experiment Setup**:
- You have four potato cups labeled A, B, C, and D.
- Cup A is kept empty, Cup B contains sugar, Cup C contains salt, and Cup D contains sugar from a boiled potato.

2. **Purpose of Each Cup**:
- Cup A (empty): Serves as a control to observe the natural state without any solute.
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