根据《地规》第5.3.5条及表K.0.3

$z_{1}/r=\frac{9}{9/2}=2$ ，查《地规》表K.0.3得 $\overline{\alpha}_{1}=0.658$

$z_{2} / r = \frac{9 + 2.7}{9 / 2} = 2.6$ ，查《地规》表K.0.3得 $\overline{\alpha}_{2} = 0.560$

$$ s = \psi_ {\mathrm{s}} \sum_ {\mathrm{i} = 1} ^ {\mathrm{n}} \frac {p _ {0}}{E _ {\mathrm{si}}} \left(z _ {\mathrm{i}} \overline {{\alpha}} _ {\mathrm{i}} - z _ {\mathrm{i} - 1} \overline {{\alpha}} _ {\mathrm{i} - 1}\right) = 1. 0 \times \frac {9 5}{3} [ 0. 5 6 0 \times (9 + 2. 7) - 9 \times 0. 6 5 8 ] = 1 9. 9 5 \mathrm{mm} $$

因此选（B）