20. Point is that none of the other answers make sense. At the object's max height, it's velocity should be zero. And by the Doppler Effect, if there the object is not moving, then there is no difference in the frequency emitted and the frequency detected. Hence, it needs to be zero at some point. B) is incorrect because the object is accelerating due to gravity and because velocity changes, the frequency shift cannot be constant. C) is incorrect because the magnitude of the frequency shift cannot increase to zero. D) is incorrect because after the object has reached its maximum height, it velocity increases as it falls back to Earth. Therefore, the frequency shift decreases (the object's speed falls as it approaches its max height), reaches zero, and then increases (as the object picks up speed as it falls).
21. You might be given the force generated by the explosion and the length of the explosion. You would be able to calculate impulse with this information.
103. H-Bonding does not always play a bigger role in BP than MW. AAMC is not going to ask you to compare BP for two compounds with very different carbon backbone lengths. Even if they did, the passage would likely provide the necessary supplemental information to answer the question.
111. The passage provides evidence that it would not be A) because all the experiments are centered around pulmonary tissue and function. Experiment 1 uses this reasoning. The body responds to pathogens by creating antibodies. If there are antibodies in the blood, then there should be a reaction between the sera and a library of known viruses and bacteria. So you know this should be an antibody reaction to something. D) is obviously not correct now. C) is incorrect because nothing suggests this is an autoimmune issue. And like I said, kidney disease hantavirus is not likely to cause significant pulmonary complications, or at least not suggested by this passage. Antibodies do have a high level of specificity, but if the epitopes are very similar, then there can be comparable levels of affinity between two related epitopes.