Let $a _ { n }$ be the number of subsets of $\{ 1,2 , \ldots , n \}$ that do not contain any two consecutive numbers. Then\\
(A) $a _ { n } = a _ { n - 1 } + a _ { n - 2 }$\\
(B) $a _ { n } = 2 a _ { n - 1 }$\\
(C) $a _ { n } = a _ { n - 1 } - a _ { n - 2 }$\\
(D) $a _ { n } = a _ { n - 1 } + 2 a _ { n - 2 }$.