GRE - GRE General Test

In 1995 the United States National Park Service reintroduced wolves into Yellowstone National Park, from which they had been eliminated decades before by overhunting. Biologists hoped the reintroduction would return the park's mix of animals to a more natural state. After the wolves* disappearance, the population of their onetime prey, the elk. had burgeoned. Subsequently, new tree growth declined as multiplying elk browsed young trees, denuding certain areas of the park. Following the wolves" return, the elk population declined and young trees rebounded. Most scientists attribute the vegetation changes to the wolves" return. However. Patton observes that Yellowstone has not had a harsh winter since wolf numbers reached high levels and suggests that elk may not have needed to resort to trees for food.

If the view attributed to the majority of scientists is correct, which of the following must be true?

Scientists have long debated the exact timing of the lunar cataclysm, a period approximately 4 billion years ago when Earth and the Moon were pummeled with asteroids. A clue to this puzzle may come from spherules, millimeter-sized droplets of molten rock formed after an asteroid collides explosively with a planet. Upon impact, the asteroid vaporizes both itself and the target rock. producing a vapor plume that condenses into spherules. These form a layer preserved in rock, whose age can be estimated using radiometric dating. Scientists know of fourteen of these spherule layers scattered across Earth, but none dates to the theorized lunar cataclysm time period. Four layers, however, are from between 3.47 and 3.24 billion years ago. indicating perhaps a slow decline in collisions.

Which of the following might plausibly account for the findings in the highlighted sentence?

One reason researchers have long believed that Mars never enjoyed an extensive period of warm and wet climate is that much of the surface not covered by wind-borne dust appears to be composed of unweathered material. If water flowed for an extended period, researchers reasoned, it should have altered and weathered the volcanic minerals, creating clays or other oxidized, hydrated phases (minerals that incorporate water molecules in their crystal structure).

It turns out. though, that the scientists were not looking closely enough. New high-resolution mapping data and close-up surface studies have revealed clays and other hydrated minerals in many regions The clay deposits are scattered all over, in ancient volcanic surfaces and heavily cratered highland regions, some of which have apparently been exposed by erosion only recently.

Consider each of the choices separately and select all that apply.

The passage indicates which of the following about the clay deposits an Mars?