[A]isbeing[B]are[C]havebeen[D]is

[A] is being [B] are [C] have been [D] is

正确答案：B

此题属于语法题本句中的主语Twothirdsofthepopulation应视为复数讲述的是一般事实故用一般现在时态故选项[B]are正确

全句可译为：2／3的美国人参加了医疗保险