I________all the cooking for my family，but recently I've  been too busy to do it.  

A．will do

B．do

C．am doing

D．had done

在墙上挂像框时，可自制一个重锤线来检查像框是否挂正，如图所示．重垂线是利用了______的原理制成的．判断像框是否挂正的标准是______．