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【单选题】
Opportunity cost is:A.
always incurred when you choose between various alternatives
B.
the cost of alternatives foregone
C.
the difference between marginal cost and average cost
D.
both (a) and (b) are correct
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【简答题】Bill: Do you often hear young Jane singing in her room?Mary: ______
【简答题】1985年开始我国确定每年的_______为教师节。
【单选题】--do you often hear Lucy ________in her room? -- Yes, listen! Now we can hear ________her in her room.
A.
sing; sing
B.
singing; singing
C.
singing; sing
D.
sing; singing
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