Correct Answer: C

Rationale: The author's primary purpose in writing the essay is to inform the reader about how an electrocardiograph reads the electric currents emitted by the heart. Choice A is incorrect because there is no attempt to persuade the reader to have an ECG. Choice B is incorrect because the essay is focused on providing information rather than entertaining with a heartwarming story. Choice D is incorrect as the essay does not primarily analyze the difference between the SA node and the AV node; instead, it focuses on explaining how an electrocardiograph reads electric currents from the heart.

Correct Answer: D

Rationale: In the context provided, 'popular' refers to magazines intended for the general public. The paragraph mentions that Infotrac indexes hundreds of popular magazines and newspapers, including medical journals such as the Journal of the American Medical Association and New England Journal of Medicine. These magazines target a wide audience, making choice D ('for the general public') the correct answer. Choices A, B, and C are incorrect as they do not align with the context of the paragraph.

Correct Answer: C

Rationale: The correct summary of the passage is option C. The passage explains how the ECG systematically measures and records the 'wave of excitation' that occurs within the heart in three distinct steps, spreading from the SA node to the AV node. This summary effectively captures the main focus of the passage. Option A is incorrect because it only mentions the detection of electric currents in the body and recognizing abnormalities by medical staff, missing the key information about the 'wave of excitation' and its stages. Option B is incorrect as it focuses solely on the measurement and recording of unique electric currents on the body's surface, neglecting the specific process described in the passage. Option D is also incorrect as it provides a general description of how the ECG measures electric currents without emphasizing the specific stages of the 'wave of excitation' as detailed in the passage.

Correct Answer: B

Rationale: Sound waves that are farther apart have a lower pitch. The Doppler Effect explains that as objects emitting sound move away, the sound waves become more spaced out, resulting in a lower frequency and a lower pitch. Choice A is incorrect because waves that are closer together have a higher frequency and a higher pitch. Choices C and D are not directly related to determining the pitch of sound waves.

Correct Answer: A

Rationale: In the context of the immune system, 'enlist' means to call into service. Immune cells recruit other cells to fight infection by producing chemicals that regulate their growth, behavior, and direct new recruits to trouble spots. This process is essential for the immune system's defense mechanism. The other choices are incorrect: 'write down' and 'put across' do not capture the action of mobilizing immune cells, and 'send away' is opposite in meaning to what is happening in the immune response.