在一个减法算式里被减数+减数+差的和是280,差比减数少20,那么被减数是( ),减数是( ),差是( )。
140;80;60
(2013·甘肃兰州、靖远名校高三模拟)Social practice is an opportunity for students to gain________better understanding of how________society operates.
A.the;the
B.the;/
C.a;/
D.the;a
m阶B+树的每个结点至多有()个子女。