You can never imagine what great difficulty I had ________ his parents.

A．finding

B．of finding

C．to find

D．about finding

拉深凸、凹模之间的间隙对拉深力、零件质量、模具寿命都有影响。间隙小，拉深力大，零件表面质量差，模具磨损大，所以拉深凸、凹模的间隙越大越好。