某自重湿陷性黄土场地，采用人工挖孔端承桩，黄土浸水的自重湿陷，使桩产生负摩阻力，已知桩顶位于地下3m，计算中性点位于桩顶下3m，黄土γ＝15.5kN/m³，w＝12.5%，e＝1.06，试估算黄土对桩产生的负摩阻力。

It’s a difficult skill to master, but ______, it will benefit you for the rest of your life.

A．once obtained

B．when to obtain

C．after obtaining

D．while obtaining