$l_0=0.9\times17=15.3\mathrm{m}$，$\gamma_m=\frac{4\times(18.5-10)+3\times(17.5-10)+6\times(18.8-10)+2.3\times(19-10)}{15.3}=8.5\mathrm{kN/m^3}$，$\sigma_1'=250+2\times18.5+0.5\times4\times8.5=304\mathrm{kN/m^2}$，$q_{s1}^n=0.3\times304=91.2\mathrm{kN/m^2}$，取$40\mathrm{kN/m^2}$；$\sigma_2'=250+(2\times18.5+4\times8.5)+0.5\times3\times7.5=332.25$，$q_{s2}^n=0.2\times332.25=66.45$，取$20$；$\sigma_3'=250+(2\times18.5+4\times8.5)+3\times7.5+0.5\times6\times8.8=369.9$，$q_{s3}^n=0.3\times369.9=110.97$，取$60$；$\sigma_4'=250+(2\times18.5+4\times8.5)+3\times7.5+6\times8.8+0.5\times2.3\times9=406.65$，$q_{s4}^n=0.35\times406.65=142.33$，取$80$。$q_{sn}=\frac{4\times40+3\times20+6\times60+2.3\times80}{15.3}=50\mathrm{kN/m^3}$，$\eta_n=\frac{s_{ax}\cdot s_{ay}}{\pi d\left(\frac{q_{sn}}{\gamma_m}+\frac{d}{4}\right)}=\frac{2.4\times2.1}{0.7\times\pi\times\left(\frac{50}{8.5}+\frac{0.7}{4}\right)}=0.38$，选A。