Which of the following functions are solutions of the differential equation $y'' - 2 y' + 1 y = 0$?

A. $y(x)=x$
B. $y(x)=xe^{-x}$
C. $y(x)=x^2e^x$
D. $y(x)=xe^x$
E. $y(x)=0$
F. $y(x)=e^{-x}$
G. $y(x)=e^x$