If \(\vec u = a\hat i + \;2\hat j - \;\hat k\;and\;\vec v = 3\hat j + 4\hat k\) are two vectors such that \(\left| {\vec u + \;\vec v} \right| \le 10\) then a = ?

• A. \(\pm 2\sqrt 3\)
• B. \(\pm\sqrt 3\)
• C. \(\pm 2\sqrt 5\)
• D. \(\pm \sqrt 5\)