X3 = | a-1|, and the period of the sequence {xn} is 3,
∴x4=|x3-x2|=||a- 1|-a|=x 1= 1，
Solution: a= 1 or a=0,
∵a≠0,∴a= 1，
∴x 1= 1,x2= 1,x3=0；
X1+x2+x3 = 2;
Similarly, x4= 1, x5= 1, x6=0,
x4+X5+X6 = 2；
…
x 20 1 1+x 20 12+x 20 13 = 2；
X20 14=x 1= 1，20 14=67 1×3+ 1，
∴s20 14=x 1+x2+x3+…+x20 14
=67 1×( 1+ 1+0)+ 1
= 1343.
So choose D.