(2020 年新高考全国卷I数学高考试题 (山东)) 已知 a>0,b>0a > 0,b > 0a>0,b>0 ,且 a+b=1a + b = 1a+b=1 ,则（ ）A. a2+b2≥12{a}^{2} + {b}^{2} \geq \frac{1}{2}a2+b2≥21B. 2a−b>12{2}^{a - b} > \frac{1}{2}2a−b>21C. log2a+log2b≥−2{\log }_{2}a + {\log }_{2}b \geq - 2log2a+log2b≥−2D. a+b≤2\sqrt{a} + \sqrt{b} \leq \sqrt{2}a+b≤2