- Mr Smith, please teach me how to draw on the computer today.

- Oh, no._______. You have to get familiar with the keyboard first.

A．Learn to walk before you run

B．It's never too old to learn

C．A good beginning is half done

D．The early bird catches the worm

区别中幼、晚幼、杆状核粒细胞时，最重要的标志是（）

A.胞体直径的大小

B.核染色质粗细情况

C.细胞核的凹陷程度

D.胞质中颗粒的多少

E.核质比的大小