Final answer:
The Cenozoic era was a time of mountain building due to tectonic activity.
Step-by-step explanation:
The correct answer is a. mountain building. The Cenozoic era, which began about 66 million years ago, witnessed significant mountain building events due to tectonic activity. One example of this is the formation of the Himalayas due to the collision between the Indian and Eurasian plates.
While continental breakup did occur in earlier geological eras, it is not specifically associated with the Cenozoic era. Global warming as we understand it today was not a significant characteristic of the Cenozoic era. Although there were some extinctions during this time, they were not necessarily slow or global in scale.
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