Which one of the following combinations of source and screen would produce sharpest shadow of an opaque object?

Which one of the following colours may be obtained by combining green and red colours ?

Which one among the following is the correct focal length of a combination of lenses of power 2.5 D and -2.0 D ?

Which one of the following colours is seen in the middle of a Rainbow?

In order to avoid sunlight, which of the following colour combination for an umbrella is most appropriate?

Which one of the following statement is not correct?