In this question, $x _ { 1 } , x _ { 2 } , x _ { 3 } , \ldots$ is an arithmetic progression, all of whose terms are integers.
Let $n$ be a positive integer. If the median of the first $n$ terms of the sequence is an integer, which of the following three statements must be true?
I The median of the first $n + 2$ terms is an integer.
II The median of the first $2 n$ terms is an integer.
III The median of $x _ { 2 } , x _ { 4 } , x _ { 6 } , \ldots , x _ { 2 n }$ is an integer.