Let a₁,a₂,…,a₂₁ be an AP such that ²⁰∑ₙ₌₁ 1/aₙaₙ₊₁=4/9. if the sum of this AP is 189, then a₆a₁₆ is equal to :
A. 36
B. 48
C. 72
D. 57