Astronomers are confident that a specific nearby volcano will eventually erupt. They are much less confident, however, about exactly when this will happen, since eruption timing depends on deep underground pressure shifts that are impossible to measure directly. Geologists recently tried to use satellite surface measurements to predict the timing but concluded that this method could not sufficiently reveal the underground pressure to allow for a precise prediction.
Which choice best describes the function of the second sentence in the overall structure of the text?
It presents the central limitation or problem (uncertainty of timing) that the text discusses.
It explains how the geologists solved the problem.
It proves that volcanoes are safe.
It lists the equipment used by the astronomers.
Correct Answer: A
Choice A is the best answer. The first sentence says an eruption is certain. The second (underlined) sentence says the timing is uncertain because of unknown internal factors. This presents the main problem the scientists face.
Choice B is incorrect. They tried to solve it but "concluded that this method could not sufficiently reveal" the answer. Choice C is incorrect. It discusses eruption prediction, implying danger. Choice D is incorrect. It mentions "satellite measurements" generally, not a list of equipment.