A relation $f$ from $A$ to $B$ is a **function** if:
Aevery element of $B$ has exactly one preimage in $A$
B$A$ and $B$ are equal sets
Csome elements of $A$ have multiple images in $B$
Devery element of $A$ has exactly one image in $B$
Answer & Solution
Correct answer: D. every element of $A$ has exactly one image in $B$
Standard definition: $f: A \to B$ is a function iff every element of $A$ has **one and only one** image in $B$.
- **A** describes injectivity-from-B, not the basic function property.
- **C** describes a non-function relation.
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