How many of the following statements are correct ?
1. Good ozone is found in the stratosphere.
2. UV radiation having a lower wavelength than UV -B is almost completely absorbed by the Earth's atmosphere.
3. UV - B causes snow blindness and cataract.
4. dB is a unit to measure the thickness of ozone.

Options

A. one
B. Two
C. Three
D. Four

Answer

To determine how many of the given statements are correct, let's analyze each statement one by one.

1. **Statement 1**: "Good ozone is found in the stratosphere."
- This statement is **correct**. Ozone in the stratosphere forms the ozone layer, which protects the Earth from harmful UV radiation.

2. **Statement 2**: "UV radiation having a lower wavelength than UV-B is almost completely absorbed by the Earth's atmosphere."
- This statement is also **correct**. The Earth's atmosphere absorbs most of the UV radiation with wavelengths shorter than UV-B, which helps protect living organisms from harmful effects.

3. **Statement 3**: "UV-B causes snow blindness and cataract."
- This statement is **correct** as well. UV-B radiation can indeed cause snow blindness, which is an inflammation of the cornea, and it is also linked to the development of cataracts in the eyes.

4. **Statement 4**: "dB is a unit to measure the thickness of ozone."
- This statement is **incorrect**. The thickness of the ozone layer is measured in Dobson Units (DU), not decibels (dB).

Now, let's summarize the findings:
- Statements 1, 2, and 3 are correct.
- Statement 4 is incorrect.

Thus, the total number of correct statements is **3**.

### Final Answer: 3