Past ice ages are believed to have been caused by

a-solar output changes and milankovitch cycles
b-volcanic activity and human activity
c-human activity and milankovitch cycles
d-plate tectonics and human activity

The correct answer for the blank is- A-solar output changes and Milankovitch cycles.

An ice age can be described as a prolonged reduction in the temperature of the atmosphere and Earth's surface, which leads to the expansion of polar ice sheets.

Fluctuations in the amount of incident rays of sunlight ( incoming sunlight) is considered as the most likely reason behind changes in Earth's climate.

In other words, alteration in the duration and the intensity of heat from the sun is the cause of glacial cycles.

The Milankovich cycles occur primarily due to changes in the shape of the Earth's orbit around the sun, which relates the tilted axis of Earth.

Thus, option A) is the right answer.

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explanation: bc blah

the most common energy conversion is from kinetic energy to potential energy, and then from potential energy to kinetic energy.

later on comes the heat energy, mechanical energy, and other energy conversions.

this energy conversion is so common that we can find it in our day to day lives as well.

- one of the most common energy conversions involves the changing of potential energy to kinetic energy or kinetic energy to potential energy