3. Given the set $P=\left\{x \mid x^{2}-5 x+4 \geqslant 0, x \in \mathbf{R}\right\}, Q=\left\{n \mid n^{2}-5 n+4 \leqslant\right.$ $\left.0, n \in \mathbf{N}_{+}\right\}$, the set $S$ satisfies $S \cap P=\{1,4\}, S \cap Q=S, S$ has the number of elements ( ).
A. 2
B. 2 or 4
C. 2 or 3 or 4
D. Infinitely many
3. C. $S$ can be $\{1,4\}, \{1,4,2\}, \{1,4,3\}, \{1,4,2,3\}$.