8. A fraction whose numerator and denominator have a greatest common divisor of 1 is called a reduced fraction. Then the sum of all positive reduced proper fractions with a denominator of 100 is ( ).
(A) 20
(B) 30
(C) 35
(D) 45
8. A.

From $100=2^{2} \times 5^{2}$, let
$$
\begin{array}{l}
A=\left\{n|2| n, n \leqslant 100, n \in \mathbf{N}_{+}\right\}, \\
B=\left\{n|5| n, n \leqslant 100, n \in \mathbf{N}_{+}\right\} .
\end{array}
$$

Then $|A \cup B|=|A|+|B|-|A \cap B|$
$$
=50+20-10=60 \text {. }
$$

Therefore, there are 40 proper fractions that meet the condition:
$$
\text { Also, }(i, 100)=1 \Rightarrow(100-i, 100)=1 \text {. }
$$

Hence, the proper fractions can be paired up.
Thus, the sum of all proper fractions is 20.