$5$ students of a class have an average height $150\ cm$ and variance $18\ cm^2$. A new student, whose height is $156\ cm$, joined them. The variance(in $cm^2$) of the height of these six students is :
(1) 22
(2) 20
(3) 16
(4) 18
(1) 22
(2) 20
(3) 16
(4) 18