电子显微镜下观察到的烟草花叶病毒的形状是（　　）

A．球形

B．螺旋形

C．杆形

D．蝌蚪形

You have spoken very good English , _____  you're the beginner of learning.        [ ]

A. after all            

B. for example

C. to speak the truth    

D. for instance