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The answer is C.

The first contact inequality is-1

So a is a necessary and sufficient condition (false)

B is a necessary and sufficient condition for the original inequality to hold.

C is a necessary and sufficient condition.

D is nothing

It is not a necessary and sufficient condition that x is y, that is to say, y can overthrow x, and x cannot overthrow y.

Small-scale inequality will lead to large-scale inequality.