Oxygen and carbon dioxide travels to and from tiny air sacs in the lungs, through the walls of the capillaries, into the blood. Blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs.

I am answering under the assumption that those are cacti in the desert. Well as we know, cacti are producers they have the ability to synthesize their own food using photosynthesis which is a process that converts solar energy into glucose “food”. It has the ability to be a good source to small animals with its fruits and seeds. An example is, some insects feed off the nectar from the cacti. A second reason is that cacti are carbon sinks meaning that they take in carbon dioxide. Just to familiarize you the formula for photosynthesis is CO2 + H2O + energy equals glucose + O2 . Without carbon dioxide none of it is possible. Without cacti, CO2 levels would increase leading to bad affects.
Charles Darwin noticed that among the same species there were variations in traits based upon geographical location. These changes may have evolved from climate differences, predator/prey relationships and food sources. This particular experiment was intended to duplicate natural selection by demonstrating the concepts with artificial selection. The result was that over time, new traits could be bred for. Although there may be more than one correct answer here, the best answer is the first; desirable traits can be selected and bred in offspring.
The answer is D its the right answer because on the moon there is not oxygen for it to reach that state