In an area where DDT had been used extensively, the population of birds declined significantly because:

Options

A. earthworms in the area got eradicated
B. cobras were feeding exclusively on birds
C. many of the birds eggs laid, did not hatch
D. birds stopped laying eggs

Explanation

**Step-by-Step Solution:**

1. **Understanding the Context**: The question states that in an area where DDT (Dichloro-Diphenyl-Trichloroethane) was used extensively, the population of birds declined significantly. We need to identify the reason for this decline.

2. **Analyzing the Options**: The options provided are:
- A) Earthworms in the area got affected.
- B) Cobras were feeding extensively on birds.
- C) Many of the bird eggs laid did not hatch.
- D) Birds stopped laying eggs.

3. **Identifying the Correct Option**:
- **Option A**: While earthworms may be affected by DDT, this does not directly explain the decline in bird populations.
- **Option B**: Cobras feeding on birds may contribute to a decline, but it is not the primary reason related to DDT.
- **Option C**: This option suggests that many bird eggs did not hatch, which is a critical factor. DDT can cause eggshell thinning, leading to poor hatching success.
- **Option D**: Birds stopping to lay eggs could be a consequence of other factors but does not directly relate to DDT's effects.

4. **Explaining the Mechanism**: DDT accumulates in the food chain. Birds consume plants or insects that have absorbed DDT. This chemical can cause physiological changes in birds, particularly affecting their reproductive systems. The eggs produced can have thinner shells, making them more susceptible to breakage and preventing proper development of the embryos.

5. **Conclusion**: The most appropriate answer is **C) Many of the bird eggs laid did not hatch**, as it directly relates to the effects of DDT on bird reproduction.

---