I don't think my sister----babysit the baby today,----?

答案是A.
C和D不对,因为my sister是单数,不能用don't.
has to 是必须,一定的意思.句子分三个部分,中心部分是 my sister has to babysit the baby today 意为：我妹妹（或姐姐）今天必须照顾婴儿.如果把 has to 换成doesn't have to 则是我妹妹（或姐姐）今天不需要照顾婴儿的意思.在中心部分之前加的 I don't think 表示否认,为我不认为 . 之意.在本文中,“我”的想法是妹妹不需要照顾婴儿.如果句子换成 I don't think my sister doesn't have to babysit the baby today, 双重否定等于肯定,即我认为妹妹（或姐姐）今天必须照顾婴儿之意.双重否定是英语习惯用语中常见的,如：he doesn't like her not talking to him. 但是就本文而言,双重否定不符合习惯用法.如果要表达“我认为妹妹（或姐姐）今天必须照顾婴儿”的意思,应该直说 I think my sister has to babysit the baby today.
句子最后用 does she? 加强语气,相当于中文句尾“是吗?”的语气.一般而言,如果前一句是肯定语气,句子最后应该用否定语气；而如果前一句是否定语气,句子最后就应该用肯定语气．I don't think my sister has to babysit the baby today 是否定语气,故用肯定语气 does she? 结尾.如果原句为肯定语气的 I think my sister has to babysit the baby today,结尾就应该是否定语气的 doesn't she? 所以B不对.