Sub fields focus more on traditional breakdowns of studies.
False mount elbrus is in kabardino balkaria Russia
What you're looking for in here is the hardness: a material can scratch another material if it's harder than it.
So the desired material cannot be harder than amphibole, which has a hardness of 6.
But it has to be harder than fluorite, which has a hardness of 4.
So we're looking for a hardness of between 4 and 6 and from the table we see it's magnesite (we'll need a hard magesite) - this is the correct answer!
They could adapt to live together so that for example one starts eating only seaweed while the other starts eating only organic matter so they don't compete for food. They could also adapt like when squirrels are separated into those who eat on trees and those who eat on the ground. The 2 urchin types could split into those who eat of the ground and those that eat what floats and drifts.
The answer would be $17.70. good luck!