--- I haven’t finished my essay because the computer broke down when I was downloading the required data last night.

--- Really! You could have turned to me. I ______ mine.

A．didn’t use

B．won’t use

C．hadn’t used

D．wasn’t using

如图所示，质量分布均匀的直角三角板ABC重为20N，可绕过C点、垂直于板面的水平转动轴自由转动，A点用竖直线AD拉住，当BC处于水平平衡位置时AD线上的拉力大小为F。后将一块凹槽口朝下、重为4N的物块卡在斜边AC上，物块沿斜边AC匀加速下滑，当物块经过AC的中点时细线的拉力大小变为F＋△F，则下述结论中正确的是

A．F＝10N

B．F＜10N

C．△F＞2N

D．△F＝2N