If the vector field H = f(r) r, r ≠ 0 is solenoidal, find an expression for f(r).
◦ f(r) = c r - 2, where c is an arbitrary constant
◦ f(r) = c r -1, where c is an arbitrary constant
◦ f(r) = c r 3, where c is an arbitrary constant
◦ f(r) = c r 2, where c is an arbitrary constant
◦ f(r) = c r -3, where c is an arbitrary constant