There is a sequence an for a positive integer such that a3=a1+a2, a4=

There is a sequence an for a positive integer such that a3=a1+a2, a4=a3+a2+a1, an =an-1+an-2+an-3+.+a2+a1. If an=p, an+2=?

A. 2p B. 4p C. 8p D. 16p E. 24p