Why couldn't Tom and Judy find out who sent them the tickets？
A. Their friends wanted to keep it a secret.
B. The letter with the tickets was not signed.
C. There was no letter with the tickets.
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边际值的数学原理表示()。
A. 自变量的取值
B. 因变量的取值
C. 自变量的变化率
D. 自变量每变化一个单位，引起因变量变化多少，即因变量的变化率
The author mentions all of the following as potential food sources for bears EXCEPT:
A. berries
B. general stores
C. insects
D. fish
E. picnic baskets
该合同中，双方是否可以约定合同争议按照美国法处理？()
A. 可以
B. 不可以
OFFHAND : PREMEDITATION ::
A. upright : integrity
B. aboveboard : guile
C. cutthroat : competition
D. backward : direction
E. underlying : foundation
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