What is the author's primary purpose in writing the essay?

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: B

Rationale: In the context provided, 'emitting' means releasing, specifically referring to the electric currents being released in the body. The passage describes how electric currents are constantly being emitted in the body and can be detected on the body's surface, ultimately recorded by an electrocardiograph. This process is about the release of electric impulses, making 'releasing' the correct interpretation. Choice A, 'Repelling,' implies pushing away, which is not the intended meaning in this context. Choice C, 'Closing,' and Choice D, 'Charging,' do not align with the context of electric currents being emitted.

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: D

Rationale: The correct steps of the 'wave of excitation' are the excitation of the atrium, excitement of the ventricle, and relaxation of the ventricle. In the described process, the 'wave of excitation' starts with the discharge from the SA node, leading to the excitation of the atrium. Next, the peak ECG corresponds to the excitement of the ventricle, not its relaxation. Finally, the wave concludes with the relaxation of the ventricle, not its contraction. Therefore, choices A, B, and C are incorrect as they do not accurately describe the sequential steps of the 'wave of excitation' as presented in the extract.

Correct Answer: B

Rationale: In the context of the given extract, 'evolved' means to gradually develop. The systems in the body have evolved over time, indicating a gradual development and adaptation. 'To spread,' 'to revolve,' and 'to shift' do not capture the idea of a gradual development as portrayed in the text, making them incorrect choices.