D) 4v
Explanations:
At point A, radius is 2r and at point B, radius is r.
Area × velocity = constant
Hence πr 2 × V = constant
i.e. r1 2V1 = r2 2V2
Given r1 = 2r
r2 = r
V1 = V
V2 =?
∴ (2r)2 × V = r2 ∙ V2
∴ V2 = 4V
V2 means velocity at point B i.e.
V-B = 4V