A ladder of length 17 metres rests with one end against a vertical wall. The lower end of the ladder is pulled away from the wall at the rate of 1.5 metres/minute. The rate at which the upper end of the ladder falls when its bottom end is 8 metres away from the wall is :

$-\frac{5}{6}$ metres /minute $-\frac{4}{5}$ metres /minute $-\frac{1}{5}$ metres /minute $-2$ metres /minute

$-\frac{4}{5}$ metres /minute