下列哪种心肌细胞4期自动去极速度最大（）

A.心房肌细胞

B.浦肯野细胞

C.房室交界细胞

D.窦房结细胞

E.心室肌细胞

I don't like math, because it's ____________.                                         [ ]

A. interesting                  

B. fun    

C. relaxing                  

D. difficult