根据《桩规》第5.6.1条

$$ G _ {\mathrm{k}} = 2 0 \times 1. 5 \times 4 0 \times 1 0 = 1 2 0 0 0 \mathrm{kN} $$

$$ \xi = \frac {f _ {\mathrm{ak}} A _ {\mathrm{c}}}{F _ {\mathrm{k}} + G _ {\mathrm{k}}} = \frac {1 0 0 \times (4 0 \times 1 0 - \mathrm{n} \times 0 . 2 ^ {2})}{3 6 0 0 0 + 1 2 0 0 0} \geq 0. 6 $$

$$ n \geq \frac {F _ {\mathrm{k}} + G _ {\mathrm{k}} - \eta_ {\mathrm{c}} f _ {\mathrm{ak}} A _ {\mathrm{c}}}{R _ {\mathrm{a}}} = \frac {3 6 0 0 0 + 1 2 0 0 0 - 0 . 7 5 \times 1 0 0 \times \left(4 0 \times 1 0 - n \times 0 . 2 ^ {2}\right)}{1 5 0} $$

解得 $2800(\text{根}) \geq n \geq 122.5(\text{根})$

因此选（C）