Reason why greenhouse effect and ozone layer depletion are considered two related serious environmental concerns of the 21st century.

Iwould choose c or d

When a lake, river or other aquatic  system  becomes  eutrophic, it can have serious negative effects on other organisms like fish, birds and even people.if an algal bloom is so bad that it causes wide-spread death  in  the  water, the organisms that die will all sink to the bottom and start to decompose.

D. i believe that doesn't even exist.