Which combination of length and diameter for the second string would result in a first-harmonic (fundamental) frequency that is exactly half the first-harmonic frequency of the first string?
Length: 2L2L2L, Diameter: 2D2D2D
Length: 4L4L4L, Diameter: D2\frac{D}{2}2D
Length: LLL, Diameter: D2\frac{D}{2}2D
Length: L2\frac{L}{2}2L, Diameter: 2D2D2D