If a,b,c are real positive numbers and

θ=tan⁻¹[a(a+b+c)/bc]¹/²+tan⁻¹[b(a+b+c)/ca]¹/²+tan⁻¹[c(a+b+c)/ab]¹/²,

then tanθ equals
(a) 0
(b) 1
(c) a+b+c/abc
(d) 2