In a `Ni-Cd` battery `(` more than one correct `)`

Options

A. All the reactants and products in the overall reaction are in the solid state.
B. The voltage of the cell changes rapidly.
C. The elctrolyte used is an alkali solution.
D. All of the above are true.

Explanation

To solve the question regarding the Nickel-Cadmium (Ni-Cd) battery, we will analyze each of the statements provided and determine their correctness based on the information about the battery's chemistry and operation.

### Step-by-Step Solution:

1. **Understanding the Components of Ni-Cd Battery**:
- A Ni-Cd battery consists of cadmium (Cd) as the anode and nickel oxide (NiO2) as the cathode.
- The electrolyte used is a potassium hydroxide (KOH) solution, which is alkaline.

2. **Analyzing the Statements**:
- **Statement 1**: "All the reactants and products in the overall reaction are in the solid state."
- **Analysis**: The overall reaction involves cadmium and nickel hydroxide, which are indeed in solid form. Therefore, this statement is **correct**.

- **Statement 2**: "The voltage of this cell changes rapidly."
- **Analysis**: The voltage of a Ni-Cd battery does not change rapidly; it remains relatively stable during discharge, only decreasing slightly. Thus, this statement is **incorrect**.

- **Statement 3**: "The electrolyte uses an alkali solution."
- **Analysis**: The electrolyte is KOH, which is indeed an alkaline solution. Therefore, this statement is **correct**.

- **Statement 4**: "All of the above statements are not true."
- **Analysis**: Since we have established that Statements 1 and 3 are correct, this statement is **incorrect**.

3. **Conclusion**:
- The correct statements regarding the Ni-Cd battery are Statements 1 and 3. Statement 2 is incorrect, and Statement 4 is also incorrect.

### Final Answer:
- The correct statements about the Ni-Cd battery are:
1. All the reactants and products in the overall reaction are in the solid state. (Correct)
2. The electrolyte uses an alkali solution. (Correct)