Review this data. An objective of a scientific review of observations is to develop testable hypotheses that can explain the observations.
Below are five statements, indicate whether they are testable hypotheses to explain the decline in deaths of the swallows over time.

a) Traits that help individuals avoid being hit by cars have evolved through natural selection in the cliff swallow population.
b) An unknown element has been protecting the swallows.
c) The population size of cliff swallows living near roads has decreased over time.
d) Scavenger population sizes have increased, and more road-killed swallows get eaten before they can be found.
e) The number of cars driving on these roads has increased over time.