Which of the following statements is false?

• A
  $1$. If $(a, b, c)$ are direction ratios of a line,then $a^2+b^2+c^2 \neq 1$.
• B
  $2$. The direction cosines of a line can be its direction ratios but not vice-versa.
• C
  $3$. If $(l, m, n)$ is one set of direction cosines,then $(-l, -m, -n)$ is also a valid set.
• D
  $4$. If $(l_1, m_1, n_1)$ and $(l_2, m_2, n_2)$ are direction cosines of perpendicular lines,then $l_1 l_2+m_1 m_2+n_1 n_2=1$.