One second is defined as

Options

A. A. 1650763.73 perods of krypton clock
B. B. 652189.6 peridos of krypton clock
C. C. 1650763.73 periods of cesium clock
D. D. 9192631770 periods of cesium clock.

Explanation

To solve the question "One second is defined as", we need to identify the correct definition from the given options.

### Step-by-Step Solution:

1. **Understand the Definition of a Second**:
- The second is a unit of time that has a specific definition based on a physical phenomenon.

2. **Identify the Relevant Physical Phenomenon**:
- The second is defined based on the vibrations or oscillations of a particular atom. In modern definitions, it is based on the cesium atom.

3. **Know the Frequency of Cesium-133**:
- The frequency of the cesium-133 atom is precisely defined. It oscillates at a frequency of 9,192,631,770 Hz (or cycles per second).

4. **Analyze the Given Options**:
- Option A: 1,650,763.73 periods of krypton clock
- Option B: 652,189.6 periods of krypton clock
- Option C: 1,650,763.73 periods of cesium clock
- Option D: 9,192,631,770 periods of cesium clock

5. **Select the Correct Option**:
- Since we know that one second is defined as 9,192,631,770 periods of the cesium clock, the correct answer is **Option D**.

### Final Answer:
One second is defined as **9,192,631,770 periods of cesium clock** (Option D).

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