Question 9
TrungTiger wrote:
GMATNinjaDear Sir,
Could you please explain more for question 9?
I agree that option B undermines the author's explanation. However, I see option E also weaken the author. According to the passage, the explanation is for the quakes at "a depth of more than 50 kilometers". Therefore, if E is true, the explanation is unvalid for all cases below 650km --> undermine the author's explanation
Passage: "The question remained: how can such quakes occur, given that mantle rock at a depth of more than 50 kilometers is too ductile to store enough stress to fracture? Wadati’s work suggested that deep events occur in areas (now called Wadati-Benioff zones) where one crustal plate is forced under another and descends into the mantle. The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much more liable to fracture."
9. The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
(A) Deep events are far less common than shallow events.
(B) Deep events occur in places other than where crustal plates meet.
(C) Mantle rock is more ductile at a depth of several hundred kilometers than it is at 50 kilometers.
(D) The speeds of both P and S waves are slightly greater than previously thought.
(E) Below 650 kilometers earthquakes cease to occur.
The passage tells us about two types of earthquakes: those that take place "a few kilometers" from the surface, and some that take place "hundreds of kilometers down in the Earth’s mantle."
The author offers an explanation for the deep earthquakes, saying that they occur "where one crustal plate is forced under another." As you've noted, (B) undermines this explanation.
Take another look at the exact wording of (E):
Quote:
(E) Below 650 kilometers earthquakes cease to occur.
(E) doesn't tell us anything about the author's explanation. It just gives us a range of where these earthquakes occur: somewhere above 650 kilometers in depth. So, it's not that the author's explanation is "invalid" for earthquakes below that depth -- earthquakes just don't happen
at all below that depth. The author's explanation could still be perfectly valid for all deep earthquakes that actually occur.
Eliminate (E) for question 9.
I hope that helps!