Miss Pei is on time for everything. How ______ it be ______ she was late for the opening ceremony?

A．must; where

B．can; that

C．should; when

D．might; if

用弹簧测力计可以测量滑动摩擦力的大小如图所示，通过弹簧测力计沿水平方向拉物块，使物块做________运动（测力计示数见图），根据_______知识，物块与桌面之间的摩擦力大小为_________.