Cho a > 0, b > 0 thỏa mãn \({a^2} + {b^2} = 7ab\). Khi đó \({\log _2}a + {\log _2}b\) bằng:

A. \(\frac{1}{2}{\log _2}\left( {\frac{{a + b}}{3}} \right)\)

B. \(4{\log _2}\left( {\frac{{a + b}}{6}} \right)\)

C. \(2{\log _2}\left( {a + b} \right)\)

D. \(2{\log _2}\left( {\frac{{a + b}}{3}} \right)\)