What may be the oldest fossil footprint yet found was discovered in June 1968 by William J. Meister, a non-professional fossil collector.

A．a part-time

B．a spare-time

C．an untrained

D．an amateur

“受电弓动态包络线”是指，它是指受电弓在运行过程中可能达到的（）

A.最大允许抬升量的轮廓线

B.最大允许摆动量的轮廓线

C.A和B全部