5. Given $P=-\frac{1}{201603 \times 201604} \quad, \quad Q=-\frac{1}{201602 \times 201604}$, $R=-\frac{1}{201602 \times 201603}$, then the size relationship of $P, Q, R$ is ( ).
(A) $P>Q>R$
(B) $P>R>Q$
(C) $Q>P>R$
(D) $R>Q>P$
Answer: A.
Solution: Let $201603=a$, then
$$
\begin{array}{c}
P=-\frac{1}{a(a+1)}=-\frac{1}{a^{2}+a}, \\
Q=-\frac{1}{(a-1)(a+1)}=-\frac{1}{a^{2}-1}, \\
R=-\frac{1}{(a-1) a}=-\frac{1}{a^{2}-a} .
\end{array}
$$

Obviously, $a^{2}+a>a^{2}-1>a^{2}-a>0$, (where $a=201603$).
Thus
$$
\begin{array}{c}
\frac{1}{a^{2}+a}-\frac{1}{a^{2}-1}>-\frac{1}{a^{2}-a},
\end{array}
$$

Therefore

That is
$$
P>Q>R \text {. }
$$

Hence, the answer is A.