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[主观题]
John______away for the last two days but he should be back tomorrow.A.isB.wentC.has beenD.
John______away for the last two days but he should be back tomorrow.
A.is
B.went
C.has been
D.has gone
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John______away for the last two days but he should be back tomorrow.
A.is
B.went
C.has been
D.has gone
描述某系统输出y1(t)和y2(t)的联立微分方程为
y'1(t)+y1(t)-2y2(t)=4f(t)
y'2(t)-y1(t)+2y2(t)=-f(t)
将下列DNA分子按熔链温度由低至高列序:
Ⅰ.A A G T T C T C T G A A
T T C A A G A G A C T T
Ⅱ.A G T C G T C A A T G C A G
T C A G C A G T T A C G T C
Ⅲ.G G A C C T C T C A G G
C C T G G A G A G T C C
记∇为差分算子,则下列不正确的是()。
A 12-∇-∇=∇t t t Y Y Y
B2122--+-=∇t t t t Y Y Y Y
C k t t t k Y Y Y --=∇
D t t t t Y X Y X ∇+∇=+∇)