根据《地处规》第7.3.3条第2款，采用《地处规》式（7.1.5-2）计算根据《地处规》第7.1.5条

$$ m = \frac {d ^ {2}}{d _ {\mathrm{e}} ^ {2}} = \frac {d ^ {2}}{(1 . 1 3 s) ^ {2}} = \frac {0 . 5 ^ {2}}{(1 . 1 3 \times 1 . 5) ^ {2}} = 0. 0 8 7 $$

$$ f _ {\mathrm{spk}} = \lambda m \frac {R _ {\mathrm{a}}}{\mathrm{A} _ {\mathrm{p}}} + \beta (1 - m) f _ {\mathrm{sk}} $$

$$ \beta = \frac {f _ {\mathrm{spk}} - \lambda m \frac {R _ {\mathrm{a}}}{\mathrm{A} _ {\mathrm{p}}}}{(1 - m) f _ {\mathrm{sk}}} = \frac {1 0 0 - 1 . 0 \times 0 . 0 8 7 \times \frac {1 5 0}{0 . 2 5 \times 3 . 1 4 \times 0 . 5 ^ {2}}}{(1 - 0 . 0 8 7) \times 7 0} = 0. 5 2 $$

因此选（C）。