if \(\left| {\vec a} \right| = 7,\;\left| {\vec b} \right| = 11\) and \(\left| {\vec a + \vec b\;} \right| = 10\sqrt 3 \), then \(\left| {\vec a - \vec b} \right|\) is equal to

• A. 40
• B. 10
• C. \(4\sqrt {10} \)
• D. \(2\sqrt {10}\)