指出下列句中 主语 的中心词 1 The teacher with two of his students is walking into the classroom. 2 There is an old man coming here. 3 The useful dictionary was given by my mother last year. 4 To do today's homework without the teacher's help is very difficult. 5 Exercising half an an hour a day is good to our health. 6 where to find help is still a problem.(It is still a problem where to find help.) 7 That he won the first prize surprised us all.(It surprised us all that he won the first prize.)