Many strange new means of transport have been developed in our century, the strangest of _____ being perhaps the hovercraft(气垫船).

-- Come on, please give me some ideas about the project. -- sorry, with so much work _____ my mind, I almost break down.

My aunt sang a few old songs, my uncle _______ the piano to accompany her..

It was a pity that the great writer died ______ his work unfinished.

Almost all of his time ______ experiments, he has no time to visit his parents.