$5$ 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