If the $n$ terms $a _ { 1 } , a _ { 2 } , \ldots , a _ { n }$ are in arithmetic progression with increment $r$, then the difference between the mean of their squares and the square of their mean is\\
(A) $\frac { r ^ { 2 } \left( ( n - 1 ) ^ { 2 } - 1 \right) } { 12 }$\\
(B) $\frac { r ^ { 2 } } { 12 }$\\
(C) $\frac { r ^ { 2 } \left( n ^ { 2 } - 1 \right) } { 12 }$\\
(D) $\frac { n ^ { 2 } - 1 } { 12 }$