Which of the following are true about glacial periods (i.e. ice ages) and interglacial periods on earth?

a. Interglacials have occurred approximately every hundred-thousand years during the last million years.
b. All of recorded human history has occurred during the current interglacial period which has lasted approximately ten-thousand years
c. The earth has been in ice ages during most of the time during the last million years
d. Interglacials are defined as the time period between ice ages on earth.