口袋中放有大小相等的2个红球和1个白球，有放回地每次摸取1个球，定义数列{a_n}：若第n次摸到红球，a_n=﹣1；若第n次摸到白球，a_n=1．如果S_n为数列{a_n}的前n项和，那么S₇=3的概率为（   ）

A：$C_7^5\times {\left(\frac{1}{3}\right)}^2\times {\left(\frac{2}{3}\right)}^5$

B：$C_7^5\times {\left(\frac{1}{3}\right)}^2\times {\left(\frac{1}{3}\right)}^5$

C：$C_7^3\times {\left(\frac{1}{3}\right)}^2\times {\left(\frac{2}{3}\right)}^5$

D：$C_7^2\times {\left(\frac{2}{3}\right)}^2\times {\left(\frac{1}{3}\right)}^5$