A photographic plate is placed directly ...

A photographic plate is placed directly in front of a small diffused source in the shape of a circular disc. It takes 12 s to get a good exposure. If the source is rotated by `60^@` about one of its diameters, the time needed to get the same exposure will be

A) 6s

B) 12s

C) 24s

D) 48

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