单选题：一单缝宽度a=lxl0-4m，透镜焦距为f=0.5m，若用λ=400nm的单色平行光垂直入射，中央明纹的宽度为（　　）。

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题目内容：
一单缝宽度a=lxl0-4m，透镜焦距为f=0.5m，若用λ=400nm的单色平行光垂直入射，中央明纹的宽度为（　　）。 A．2x10-3m
B．2x10-4m
C．4×10-4m
D．4x10-3m

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答案解析：

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