已知向量 a→,b→\overrightarrow{a},\overrightarrow{b}a,b 满足 ∣a→∣=1,∣b→∣=3,∣a→−2b→∣=3\left| \overrightarrow{a}\right| = 1,\left| \overrightarrow{b}\right| = \sqrt{3},\left| {\overrightarrow{a} - 2\overrightarrow{b}}\right| = 3a=1,b=3,a−2b=3 ，则 a→⋅b→=\overrightarrow{a} \cdot \overrightarrow{b} =a⋅b= （）A. -2B. -1C. 1D. 2