The pleistocene epoch was characterized by a) a mass extinction. b) the repeated advance and retreat of ice sheets. c) a warm, humid climate. d) a rapidly warming climate.

The repeated advance and retreat of ice sheets

The Pleistocene Epoch was a period of Ice Ages with a series of glacials (cooler periods, i.e. ice ages) and interglacials (warmer periods). The continents have already spread out to their present positions. However, during the cool periods, much of the temperate regions of the world were covered by glaciers, and during the warmer periods, these glaciers retreated. Human species, the Homo sapiens, also evolved and spread to different parts of the world during this time period. 

Called for cell.

The right answer is b hydrogen bonds!