For the Stoke's law to be valid in the case of a falling sphere in a fluid, the

ANS:C - sphere must be metallic.

the statement "sphere must be metallic" is not accurate or necessary for Stoke's law to be valid. Stoke's law is a principle that describes the drag force experienced by a solid sphere moving through a viscous fluid at low Reynolds numbers. The material composition of the sphere, whether metallic or non-metallic, does not affect the validity of Stoke's law. Stoke's law applies to any solid sphere moving through a viscous fluid, regardless of its material composition. The key factors determining the applicability of Stoke's law are the low Reynolds number regime (Re << 1) and the assumption of laminar flow around the sphere. As long as these conditions are met, Stoke's law accurately predicts the drag force experienced by the sphere. Therefore, the material of the sphere being metallic or non-metallic is not a criterion for the validity of Stoke's law. The key requirement is that the flow around the sphere is laminar and that the Reynolds number is much less than 1.