此句应该用therefore,
表示“因此,所以”时so是连词,可以连接两个简单句；而therefore是副词,准确的说是连接性副词,不能直接用来连接两个简单句,它可以放在句首,后面加逗号,也可以放在句中作为插入语.另外,and therefore 相当于so.还有therefore常表示很强的推断,用法比较正式,而so则常用于口语中.试比较下面的句子
He is kind; therefore,/ thus we all like him.
He is kind ,so we like him.
He is kind .Therefore/ Thus we like him.