Answer:
First parts only: Not accustomed to unit of math teachings
B: It's more effective to use colored chalk
C: Its difficult to memorize all of these relations
D: It's essential to distinguish the elements of the elements of a set from the "non-elements."
E: It's important to point out that elements of a set need be individual, but may themselves be sets.
F: It's impossible to determine the exact image in that case.
G: It's desirable to have a more simplified system of notation.
H: It's clear to see that the meaning of an expression depends on its context.
I: It's compulsory to give your full name.
J: It's necessary to do the measuring as accurately as possible.
K: Its permissible to cut [=] into 2 or 3 parts.
It's impossible to read your writing.
It's useful to use a heating pad.
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It's silly to get upset over small things.
It's difficult to solve this equation.
It's important for your health to drink lots of liquids.
Its unfair to criticise him.