What is the recursive rule for this geometric sequence? 7, 21, 63, 189, ... Responses a1=21;an=7an1 a subscript 1 end subscript equals 21 semicolon a subscript n end subscript equals 7 times a subscript n minus 1 end subscript a1=7;an=1/3an1 a subscript 1 end subscript equals 7 semicolon a subscript n end subscript equals fraction 1 over 3 end fraction times a subscript n minus 1 end subscript a1=3;an=7an1 a subscript 1 end subscript equals 3 semicolon a subscript n end subscript equals 7 times a subscript n minus 1 end subscript a1=7;an=3an1 a subscript 1 end subscript equals 7 semicolon a subscript n end subscript equals 3 times a subscript n minus 1 end subscript