$$\left\{ {\left( {\sqrt {72}- \sqrt {18} } \right) \div \sqrt {12} } \right\}$$is equal to = ?

• A. $\sqrt 6 $$
• B. $\frac{{\sqrt 3 }}{2}$$
• C. $\frac{{\sqrt 2 }}{3}$$
• D. $\frac{{\sqrt 6 }}{2}$$