TT gwd-30-14,15

A certain computer program generates a sequence of numbers a1, a2, … , an such that a1 = a2 = 1 and ak = ak-1 + 2ak-2 for all integers k such that 3 ≤ k ≤ n.  If n > 6, then a7 = ?

A.          32

B.          43

C.          64

D.          100

E.          128

我算的是53，没有这个答案

a1=a2=1

a3=a2+2a1=3

a4=a3+2a2=5

a5=a4+2a3=5+2*3=11

a6=a5+2a4=11+2*5=21

a7=a6+2a5=21+2*11=43

谢谢