There _____ some shoes under the bed.[ ]
A. is
B. are
C. am
答案：B
In ____late 19th century, computers changed _____great deal and were widely used______international trade.
A．the; a; the
B．the; the;不填
C．the; a; an
D．the; a; 不填
将用15N标记的一个DNA分子放在含有N的培养基中让其复制三次，则含有15N的DNA分子占全部DNA分子的比例和含有N的脱氧核苷酸链占全部DNA单链的比例依次是[ ]
A．1/2、1/4
B．1/4、1/8
C．1/4、1/16
D．1/8、1/8
链占全部DNA单链的比例依次是[ ]