波长为λ的单色平行光,垂直照射到宽度为 b 的单缝上,若衍射角φ=30°时,对应的衍射图样为第一级极小,则缝宽 b 为
A. λ/2
B. λ
C. 2λ
D. 3λ
___________ students may set up a lion dance.
American
B. Korean
Chinese
D. British
The Paper Scissors Stone Club was founded in ____ .
A. Paris
B. Tokyo
C. Beijing
D. London
In China,the earliest printed poster appeared in_____ Dynasty.
A. Ming
B. Song
C. Qing
D. Tang