Question

living organisms. as ancient bacteria evolved, developing the ability to perform photosynthesis, oxygen levels in earth’s atmosphere gradually increased. the graph below shows a period of time during which two sharp increases in oxygen levels in the atmosphere occurred
organism body volume during past 4 billion years
scientists think that these relatively drastic increases in oxygen levels were the result of rapid increases in photosynthetic populations of bacteria in oceans. as the graph shows, following both sharp increases in oxygen levels, there were increases in the body volume (size) of organisms. organisms use oxygen to perform cellular respiration to produce energy needed for growth and reproduction. scientists have concluded that the increase in oxygen available allowed for body
use the drop - down menus to complete this statement about how oxygen in the atmosphere and body volume are related
the sharp increases in oxygen levels in the atmosphere were the result of increasing
increased oxygen levels in the atmosphere allow for increased this allows for multicellular organisms to increase in size and energy output

Explanation:

Step1: Analyze first drop - down

The sharp increases in oxygen levels in the atmosphere are related to the process that produces oxygen. Photosynthetic organisms (like bacteria in oceans) perform photosynthesis which produces oxygen. So the first drop - down should be related to photosynthetic activity. The key here is that the increase in oxygen is due to increasing photosynthetic activity (e.g., the number of photosynthetic organisms or their rate of photosynthesis). So the first blank is filled with something like "photosynthetic activity" (or the number of photosynthetic organisms, etc.).

Step2: Analyze second drop - down

Increased oxygen levels allow for increased cellular respiration. Because organisms use oxygen in cellular respiration to produce energy (ATP) which is needed for growth and reproduction. So the second drop - down is "cellular respiration".

Answer:

First drop - down: photosynthetic activity (or equivalent related to photosynthesis - driven oxygen production)
Second drop - down: cellular respiration