For miles around me ,there was nothing but a desert, without a single plant or tree        .

A．on earth

B．in sight

C．for distance

D．at place

切片机刀片在使用过程中，如刀口磨钝或出现缺口，必须换下重新磨削，经多次磨削后，刀片高度缩短，为了能安全切削，应何调整，调整到何值？