Why is an experiment called a controlled study?

Which of the following observations darwin shape his concept of descent with modification? a) species diversity declines farther from the equator.b) fewer species live on islands than on the nearest continents.c) birds can be found on islands located farther from the mainland than the birdsê¹ maximum nonstop flightdistance.d) south american temperate plants are more similar to the tropical plants of south america than to thetemperate plants of europe.e) earthquakes reshape life by causing mass extinctions.