To keep the species from dying off.
I think your answer would be b.
Answer: D. It can melt to a depth of a few meters in summer, then refreeze in winter.
Explanation:
Permafrost is ground which remains frozen for two or more years. Permafrost consists of soil, rock and sand fused together by ice. Mostly found in areas in high altitude, the Arctic, Antarctica, it is common in periglacial environments where temperatures are below freezing
.
The topmost layer of permafrost can melt in summer and refreeze in winter. This thawing and refreezing produces a ground pattern called columnar joints similar to the one formed when molten lava cools.
Well if you mean what does the controlled part of a scientific investigation, it is the part of the investigation that will be your control or your base, untouched, original, data and such.
