The answer is "b. several million".
Pleistocene is the period in the history of Earth that we usually allude to as the Ice Age. Through quite a bit of this period, the Earth's northern and southern locales were secured by kilometer thick icy masses. Recognize that the Pleistocene was a progression of genuine ice ages, isolated by generally short interglacial periods. The Pleistocene began 2.6 million years back and kept going until the end of the Weichsel glaciation around 11,711 years prior.
