What is permafrost?

options:

a. curly crystals of frost that form early in the morning

b. ice and snow that exist throughout the entire year

c. ice and snow that are permanently in motion

d. the upper part of the ground that remains frozen throughout the year

Answer: d. the upper part of the ground that remains frozen throughout the year

Explanation: Permafrost covers large portions of the Earth crust with almost a quarter of the land area in the Northern Hemisphere having permafrost underneath them. Permafrost is defined as a combination of soil, rocks and sand held together by ice remaining frozen all year long (though the active layer at the top doesn't always stay frozen. This layer is the active layer).

Permafrost is a thick subsurface layer of soil that remains frozen throughout the year, occurring chiefly in polar regions.

I believe it would be D.