What is the recursive rule for the sequence 1,___,6,___,36,___,216,___,…?
a. aₙ​=61​⋅aₙ₋₁​, a₁​=1
b. aₙ​=6⋅aₙ₋₁​, a₁=1
c. aₙ​=−6⋅aₙ₋₁​, a₁​=1
d. aₙ​=61​⋅aₙ₋₁​, a₁​=1