根据《混规》第6.2.10条

$$ x = \frac {f _ {\mathrm{y}} A _ {\mathrm{s}} - f _ {\mathrm{y}} ^ {\prime} A _ {\mathrm{s}} ^ {\prime}}{\alpha_ {1} f _ {\mathrm{c}} b} = \frac {3 6 0 \times (2 9 4 5 - 6 2 8)}{1 . 0 \times 1 4 . 3 \times 2 5 0} = 2 3 3. 3 \mathrm{mm} > 2 a _ {\mathrm{s}} ^ {\prime} \text {且} < \xi_ {\mathrm{b}} h _ {0} $$

$$ \begin{array}{r l} M _ {\mathrm{u}} & = \alpha_ {1} f _ {\mathrm{c}} b x \left(h _ {0} - \frac {x}{2}\right) + f _ {\mathrm{y}} ^ {\prime} A _ {\mathrm{s}} ^ {\prime} \left(h _ {0} - a _ {\mathrm{s}} ^ {\prime}\right) \\ & = 1. 0 \times 1 4. 3 \times 2 5 0 \times 2 3 3. 3 \times (6 5 0 - 7 0 - 2 3 3. 3 / 2) + 3 6 0 \times 6 2 8 \times (6 5 0 - 7 0 - 5 0) = 5 0 6. 3 \mathrm{kN} \cdot \mathrm{m} \end{array} $$

则荷载 P 作用于跨中点时，弯矩最大

$$ \frac {1}{4} P \times 8 + \frac {1}{8} \times 4 0 \times 8 ^ {2} = 5 0 6. 3 $$

得 $P_{\mathrm{max}} = 93.15\mathrm{kN}$

因此选（A）