设函数f(x)表示自然数x的数字和(如:x=123,则f(x)=1+2+3=6,即f(123)=6),则方程x+f(x)+f[f(x)]=2013的解集为()
A.{1979,1985,1991,1999}
B.{1979,1985,1987,2003}
C.{1979,1985,1991,2013}
D.{1979,1985,1991,2003}