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【简答题】
光在两种介质表面上反射，如图a所示．光在薄膜上下两表面的反射，如图b所示．在这两种情况，折射率n 1 、n 2 、n 3 满足什么条件有半波损失?
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【单选题】It's five years ____ my dear aunt ____ here.
A.
before; has left
B.
since; left
C.
before; is left
D.
since; had left
【单选题】— It's five years since I worked here. — _______? [ ]
A.
Have you worked here happily
B.
How long will you work here
C.
Where do you work now
D.
Do you want to work here longer
【单选题】It's five years _______ our teacher came to Jiangyan. [ ]
A.
since
B.
when
C.
although
D.
if
【单选题】It's five years since my dear aunt _____ here. [ ]
A.
left
B.
has left
C.
is left
D.
had left
【单选题】It's almost five years___we saw each other last time.
A.
before
B.
since
C.
after
D.
when
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