完成以下句子,用had to或must have been填空.

Example：He is very tired because he had to get up early this morning。He didn't get up early this morning。He must have been tired。1 He didn't come to work yesterday。He【must have been】 ill。2 He didn't come to the office this morning。He【had to】stay at home。3 I don't think she was Austrian。She 【must have been】German。4 I lost my pen so I 【had to】 buy a new one。5 He forgot his case so he 【had to】 return home。6 She didn't hear the phone。She 【must have been】sleeping。 再问： 为什么这么填？？ 再答： had to  “不得不”，后接动词原形must have been "肯定。。。" 表示一种肯定的推测，后接v-ing，名词或者形容词均可！通过词义和后接词两种方法都可判定。