Choose the correct alternative

Options

A. During the nuclear fusion reaction, two light nuclei combine to give a heavier nucleus and possibly other products `
B. In a working nuclear reactor, control rods are used to slow down neutrons
C. Fusion reaction takes place at high temperature because atoms at ionised at high temperature
D. Energy in the sun is generated mainly by fusion by hydrogen atoms

Answer

To solve the question regarding the correct alternatives related to nuclear fusion and reactors, let's analyze each option step by step:

### Step 1: Analyze Option A
**Statement:** During the nuclear fusion reaction, two light nuclei combine to give a heavier nucleus.

**Explanation:** This statement is correct. In nuclear fusion, light nuclei, such as hydrogen isotopes (deuterium and tritium), combine to form a heavier nucleus, such as helium. During this process, a small amount of mass is converted into energy according to Einstein's equation \(E=mc^2\).

### Step 2: Analyze Option B
**Statement:** In a working nuclear reactor, control rods are used to slow down.

**Explanation:** This statement is incorrect. Control rods in a nuclear reactor are made of materials like boron or cadmium, which are used to absorb excess neutrons rather than to slow them down. The slowing down of neutrons is achieved through a moderator (like water or graphite), not the control rods.

### Step 3: Analyze Option C
**Statement:** Fusion reactions take place at higher temperatures.

**Explanation:** This statement is correct. Fusion reactions require extremely high temperatures (on the order of millions of degrees Kelvin) to overcome the electrostatic repulsion between positively charged nuclei. For example, fusion in the sun occurs at temperatures around \(10^7\) Kelvin.

### Step 4: Analyze Option D
**Statement:** Energy in the Sun is generated mainly by fusion of hydrogen atoms.

**Explanation:** This statement is correct. The primary source of energy in the Sun is the fusion of hydrogen nuclei into helium, which releases a vast amount of energy in the form of light and heat.

### Conclusion
Based on the analysis:
- **Correct Alternatives:** A, C, and D.
- **Incorrect Alternative:** B.

### Final Answer:
The correct alternatives are A, C, and D.

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