Answer: B. Slower population growth
Explanation:
Slower population growth is Not a reason why communicable diseases like pneumonia kill fewer people today than they did in the past, this is because in present times irrespective of the population growth, newer approaches have been introduced to improve quality of life and hence reduce the risk of communicable diseases.
Approaches like; continuing medical advances, improved living conditions, better preventive treatment. Etc.
I believe you are asking about the "triangle" we were tought a simple triangle. Cells at the bottom because there are the most of them, tissues next, organ next, and organ system at the top. So I believe the answer you are looking for is Organ System.
