Which statement describes the best example of an observational study? O A. A scientist measures the height of different mountains in Asia.
O B. A scientist measures the height of trees before and after they are
given fertilizers.
O C. A scientist measures the speed of a river without a dam and
compares it to the speed of a river with a dam.
O D. A scientist measures the weight of bears in protected habitats and
compares it to the weight of bears outside protected habitats.