1. c. 100,000 years
There has been multiple glaciations during the geological past of the Earth, but while they have all been separated by millions of years and have appeared randomly, this is not the case in the Pleistocene. The glaciations in the Pleistocene have started to have a cyclic pattern. This pattern is approximately 100,000 years, which is actually the period from the beginning to the peak of the glaciation, thus in general the peak of the Pleistocene glaciations is in about 100,000 years since the beginning of the glaciation.
Q 68. b. Huronian
The Huronian is the first glaciation known to the scientists. It has occurred between 2.4 to 2.1 billion years ago. The reason for the appearance of this glaciation is thought to be the quietness of the volcanoes for millions of years. Because the volcanoes were not erupting, or it was very little, the CO2 levels in the atmosphere dropped, resulting in significant weakening of the greenhouse effect, sot he Earth cooled off, and became a snow/ice ball. This glaciation occurred when there was only unicellular organisms, thus before the explosion of life.r
Q 69. c. Glacial erratics are blocks of rock that are too large for the glacier to move
The glaciers are very powerful, and as they move they manage to move lot of things with them. The things that are moved by the glaciers and later deposited vary from very small sand grains to enormous blocks of rocks. The big blocks of rock that weigh several tons are called glacial erratics. Despite their weight, the glaciers were able to move them over long distance, tens or even hundreds of km away. These rocks are easy to notice, as they stick out in the landscape, having different composition that the native rocks, and looking like they don't belong there.
Q 70. c. About 35 million years ago
Antarctica is a frozen continent in the present, but this has not been the case always. In the past, Antarctica has actually been a continent that was covered with dense forests, having tropical and temperate climate types. This started to change with the breakup of Gondwanaland. As Antarctica was moving away from South America and Africa, it was moving toward the South Pole, and as it was getting closer, the climate started to change. Around 35 million years ago, the first glaciers started to form on Antarctica, and over the next 20 million years, this continent gradually was becoming more and more covered in ice until it got almost completely frozen.
Q 71. d. icy comets
The glaciers on Earth depend on numerous factors in order for them to form and exist. Some of those factors are the precession, eccentricity, and obliquity, all of which have very big impact on Earth's climate, making it warmer and wetter at times, and than making it colder and drier. When the climate is becoming colder and drier, the glaciers start to form, and they start to expand from the poles toward the lower latitudes, as well as occupying the higher mountains around the globe. The icy comets though have nothing to do with the formation of glaciers. While they may have impact with our planet and cause changes, they will be very short lived, and the ice they carry will melt down immediately at the moment of the impact.