题干
He has been in high spirits, for pieces of exciting news came to him __________.
A：one after another
B：one by one
C：little by little
D：step by ste
答案(点此获取答案解析）
A
子上至少有1个酶切位点被该酶切断，则从理论上讲，经该酶切割后，这些线性DNA分子最多能产生长度不同的DNA片段种类数是（ ）