what does language level a1 a2 b1 b2 c1 and c2 mean

what does language level a1 a2 b1 b2 c1 and c2 mean

what does language level a1 a2 b1 b2 c1 and c2 mean. There are any references about what does language level a1 a2 b1 b2 c1 and c2 mean in here. you can look below.

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