问题
选择题
A kangaroo cannot jump if its tail is ________ the ground. [ ]
A. on
B. in
C. of
D. off
答案
D
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A kangaroo cannot jump if its tail is ________ the ground. [ ]
A. on
B. in
C. of
D. off
D
的单色光从
和
投射到屏上，并且
，问P点是亮条纹还是暗条纹？设O为到