I WILL GIVE 20 EXTRA POINTS!!
How did the egg shell of either species return to their original thickness by 1980? Suggest a reason to explain your answer.

Due to DDT ban

Explanation:

DDT is an pesticide or insecticide which got banned because of its harmful effect in environment.

According  to the graph the eggshell of both the bird species A and B are thick in 1930 but begin to thin after 1940 as DDT () introduced in the ecosystem.

Eggshell return to their original thickness by 1980 because DDT got banned after 1970. DDT is a harmful pesticide which alter the bird's calcium metabolism which leads to thin eggshells.

Hence, due to banning of DDT after 1970,egg shell of either species return to their original thickness by 1980.

hammurabi rulers

good lesson

i believe it would be c: tons.