"If and only if" (abbreviated "iff") is a biconditional logical connective between two assertions that is either true or untrue. The correct option is D.
In logic and related subjects such as mathematics and philosophy, "if and only if" (abbreviated "iff") is a biconditional logical connective between two assertions that is either true or untrue. It is represented by ⇔ or ↔.
The two of the events are
Now, the representation of "the whole number has one digit if and only if the whole number is less than 10" is done as p ↔ q.
Hence, the correct option is D.
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