More than 1000 different species of cichlid fish live in Lake Malawi in Africa. Scientists studied sediment records from the lake. They found that the water levels dropped more than 200 meters around 24 times. Which of the following is the best example of how a water level drop could have contributed to the evolution of new species?
A:The fish had to adapt to a lack of oxygen.
B:The fish had to adapt to changes in the air.
C:The fish had to adapt to changes in water pH and salt levels.
D:The fish had to adapt to the sediment in the lake bed.