Required information

$...$ continuing in the same manner

$=2^{n}n(n-1)(n-2)(n-3)cdots(n-(n-1))a_{n-n}$

$=2^{n}n(n-1)(n-2)(n-3)cdots1*20$

Multiple Choice

$2^{n}n!$

$3*2^n(n-1)$

$3*2^{n}n$

$3*2^n(n+1)!$