Which formula can be used to find the nth term of the sequence 3, 21, 147, ...?
A) a₁ = 7; aₙ = aₙ₋₁ * 3
B) a₁ = 1, aₙ = aₙ₋₁ * 7
C) a₁ = 3, aₙ = aₙ₋₁ * 7
D) a₁ = 3; aₙ = aₙ₋₁ * 21