— English has a large vocabulary, hasn't it?

— Yes.________more words and expressions and you will find it easier to read and communicate.[ ]

A. Mastering  

B. To master

C. Master  

D. Having mastered

克隆羊“多莉”所进行的核移植实验使用的细胞核来自（）

A.精母细胞

B.胚胎干细胞

C.乳腺细胞

D.卵细胞

E.多能干细胞