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【简答题】
已知数列{a n }中，a 1 =3，a n+1 -2 a n =0，数列{b n }中，b n ?a n =（-1） n （n∈N * ）． （Ⅰ）求数列{a n }通项公式； （Ⅱ）求数列{b n }通项公式以及前n项的和．
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A.
It compares favorably with home-made lunch.
B.
It is unpopular among students.
C.
It is supported by most parents.
D.
It contains fewer calories and is less expensive.
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A.
external rewards decrease intrinsic motivation
B.
decreases in intrinsic interest resulting from an external reward can be detected by statistical analysis
C.
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D.
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