21. 如圖所示之周轉輪系,各齒輪齒數分別為TA=40、TB=20、TC=10、TD=80,若 ND=0,而NA=20 rpm(順時針),則旋臂m之轉向及轉速為何? (A)20 rpm(逆時針) (B)20 rpm(順時針) (C)40 rpm(逆時針) (D)40 rpm(順時針)