An extended period of well-below-average temperatures is known as a
a. ice age.
b. draft.
c. drought.
d. cold wave.

The answer is letter A.

Ice Age or glacial period refers to the extended period of well-below-average temperatures. It is a prolonged period where reduction in the temperature of the Earth’s surface and atmosphere are caused by the expansion of continental and polar ice sheets and alpine glaciers. Usually this phenomenon occurs in the northern and southern hemispheres where extensive ice sheets are seen most.

At present we are now experiencing an interglacial period after the Holocene Ice age.

An extended period of well below average temperatures is known as a (A.) ice age. 

We know it is not a draft or drought and cold wave isn't considered an extended period of time. So ice age is the only one left. That, and I am taking the test and this is what it says in the text book. :) good luck!

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.