The headmaster could not go to the meeting; therefore, Mr. Smith _______ him.

While Helen ________ the piano，her mother came in．

We had a really bad time about six monthsago but now things are ____

With the word “ PM25” _________ appearing in media reports, people pay greater attention to it and seek health tips for smoggy days

nce worked in a remote mountainvillage school, which is ____only on foot