Movie ticket prices have reached an all-time high. ________ some people choose instead to watch movies at home.
A. If anything
B. In the absence of
C. No wonder
D. No doubt
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__________ that story book was indeed very popular, but it was not long before the demand declined.
A. To a while
B. Some time
C. Sometime
D. For a while
已知R、S是N上的关系，其定义如下：R={| x,yÎN∧y=x2}，S={| x,yÎN∧y=x+1}。求R-1、R*S、S*R
证明：若f:A→B和g:B→C是双射，则（gf）-1=f-1g-1
结构钢焊接时焊条选择的主要原则是焊缝与母材在下列哪一方面应相等。
A. 化学成分
B. 结晶组织
C. 强度等级
D. 抗腐蚀能力
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