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

Question

$\begin{array}{1 1} \text{a. Many of the birds laid eggs that did not hatch due to thin shells} \\ \text{b. Birds stopped laying eggs} \\ \text{c. Earthworms in the area got eradicated} \\ \text{d. Cobras were feeding exclusively on birds} \end{array} $