题干
Liu Ming is said abroad for one year before he came to teach in our school．
A：to have studied
B：to study
C：to be studying
D：to have been studying
答案(点此获取答案解析）
A
。则该DNA分子连续复制两次共需游离的胸腺嘧啶脱氧核苷酸的数目是（ ）
