请教各位TPO15阅读第三篇阅读中的一道题目，谢谢~

mengdanUID: 2833797

One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth's crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Teriary periods shows that there si a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though 【no asteriod itself has ever been recovered.】

In stating that 【no asteriod itself has ever been recovered.】the author emphasizes which of the following?
A: The importance of the indirect evidence for a large asteriod.
B: The fact that no evidence supports the asteriod-impace hypothesis.
C: The reason many researchers reject the Alvarez hypothesis.
D: The responsibility of scientists for not making the effort to discover the asteriod itself.


答案是A，我做的时候是用排除法做出来的。大家能告诉我为什么选A么？我没有完全理解这句话的意思。先谢谢啦

hycqyUID: 2819662

1# mengdan 帮你捋一下思路 这段说的是asteriod引发iridium increasing 从而能证实Alvarez hypothesis
This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteriod itself has ever been recovered.
这句话说明了虽然iridium increasing被我们观测到了 但asteriod itself还没有被发现 也就是说asteriod itself是一个重要的证据 虽然iridium increasing已经能说明一定问题 但是如果能找到asteriod itself就更有说服力了

mengdanUID: 2833797

2# hycqy
恩，谢谢你了！！！