The sum of of infinite terms of the series cot⁻¹ (1/2+2²) + cot⁻¹(1/2² + 2³) + (1/2³ +2⁴) + ..... [infinity] is

a. π/3
b. tan⁻¹ 2
c. cot⁻¹ 2
d. π/4