_______the class, the students are all noisy. [ ]

A. Begin with            

B. At the begin    

C. At the beginning      

D. At the beginning of

如图为用道钉撬来撬铁路枕木上道钉的图片，若阻力F₂为1500N，根据图片估算此道钉撬所用最小动力F₁的大小．