— I hate talking with that guy. Look, he is coming. What should I do?

— Don’t speak until ____.

A．speaking

B．spoken to

C．spoken

D．speaking to

为了测定木块与桌面间的动摩擦因数，用原长为的弹簧，把木块吊起来，稳定后弹簧长，用这个弹簧水平拉动木块，使木块在桌面上做匀速直线运动，弹簧长为。试求木块与桌面间的摩擦因数是         。