If the first 4 terms of a geometric sequence are {7,21,63,189}, then the formula for the n^(th) term in the sequence is _________.

a.) aₙ=7· 3ⁿ⁻¹⁾
b.) aₙ=7· (n-1)³
c.) aₙ=3· (n-1)⁷
d.) aₙ=3· 7⁽ⁿ⁻¹⁾