图甲:玻璃珠被加热后,小灯泡发光.说明____;
图乙:半导体二极管D接入电路,小灯泡发光,调换电池的正负极后,小灯泡不发光.这说明____.
下列词语书写完全正确的一项是( )
The desk is not dirty. You _______ clean it.