As a result, these black nails jutted almost invisibly from the blackboard and this teacher kept banging his hands on them while erasing the board.

• 由于钉子的颜色和黑板一样，都是黑色，即使突出来也不明显的。再加上，老师边擦黑板边把钉子往里按紧，这下就更加隐蔽了。