Choose the correct statements from the following:

Options

A. Good reflectors are good emitters of thermal radiation
B. Burns caused by water at `100^@C` are more severe than those caused by steam at `100^@C`
C. If the earth did not have atmosphere, it would become intolerably cold
D. It is impossible to construct a heat engine of `100%` efficiency

Explanation

To solve the question, we need to evaluate each statement provided in the options and determine which ones are correct based on the principles of thermodynamics and calorimetry.

### Step-by-Step Solution:

1. **Evaluate the first statement: "Good reflectors are good emitters of thermal radiation."**
- According to Kirchhoff's law of thermal radiation, a good emitter of thermal radiation is also a good absorber. Conversely, a good reflector cannot be a good emitter. Therefore, this statement is **incorrect**.

2. **Evaluate the second statement: "Burns caused by 100°C water are more severe than those caused by 100°C steam."**
- The severity of burns depends on the energy transferred to the skin. Steam at 100°C contains latent heat, which means it can release more energy upon condensation than water at the same temperature. Thus, burns caused by 100°C steam are indeed more severe than those caused by 100°C water. Therefore, this statement is **incorrect**.

3. **Evaluate the third statement: "If Earth did not have an atmosphere, it would become intolerably cold."**
- The atmosphere acts as a blanket, retaining heat during the night. Without it, the Earth would lose heat rapidly at night, leading to much colder temperatures. Therefore, this statement is **correct**.

4. **Evaluate the fourth statement: "It is impossible to construct a heat engine of 100% efficiency."**
- According to the second law of thermodynamics, it is impossible to create a heat engine that operates at 100% efficiency because some energy is always lost as waste heat. While theoretically, a Carnot engine can achieve 100% efficiency under ideal conditions, practically, it is impossible. Therefore, this statement is **correct**.

### Summary of Correct Statements:
- The third statement is correct: "If Earth did not have an atmosphere, it would become intolerably cold."
- The fourth statement is also correct: "It is impossible to construct a heat engine of 100% efficiency."

### Final Answer:
The correct statements are:
- "If Earth did not have an atmosphere, it would become intolerably cold."
- "It is impossible to construct a heat engine of 100% efficiency."