Some invertebrates in antarctica have which compound in their blood to protect against the cold

The correct answer is glycerol. This is what animals like seals have. It's basically a bunch of fat tissue that is found under the skin in large quantities to isolate the body and protect it from the immense cold. Seals aren't the only ones as there are many animals in the polar regions who have it.

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.

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