Ans: A
$f(x)<3$ means the part of $y=f(x)$ under the line $y=3$. Therefore, the solution of $f(x)<3$ is $a < x < d$.
$f(x)<3$ means the part of $y=f(x)$ under the line $y=3$. Therefore, the solution of $f(x)<3$ is $a < x < d$.
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