单选题 . ①Pikas are small, rabbit-like mammals that live high in the mountains on rocky slopes in western North America. ②During the 1990s, ecologist Eric Beever revisited 24 locations—all in the Great Basin of the western United States—where pikas had been observed between 1898 and 1947, and he found that seven of the original sites no longer had pikas. ③The pika populations that had vanished were those at lower, warmer elevations, which suggests climate warming is involved. ④Generally, when climate warming changes habitat, animals move either to higher, colder elevations or farther north. ⑤However, Pikas in the Great Basin are not easily able to migrate in this way. ⑥They live on mountain ranges that are separated from other mountains by inhospitable valleys. ⑦Further, even the most widely moving pikas tend to move less than one kilometer from their birthplaces during their lifetimes.21. Consider each of the choices separately and select all that apply.
Which of the following, if true, would strengthen the argument that climate change was involved in the disappearance of certain Pika populations? ______

A、 Outside the Great Basin, many Pika populations are dwindling.
B、 Some plants that used to be part of Pika habitats at lower elevations are now found only at higher elevations.
C、 Certain other animal populations in the Great Basin region have actually experienced modest increases in numbers.

【正确答案】 B

【答案解析】 本题属于加强题。需要加强的论证是：气候变暖→鼠兔减少。
B：属于列举证据。通过说明低海拔的植物也消失了来进一步加强气候变暖这一原因。
A：看不出是因为什么导致鼠兔减少，属于无关选项。
C：数量上升，削弱原文。