Though suffering constant noise from the outside world, little Tommy was still working with all his attention _____ on his unfinished warship model.

“The day is coming ________ telegraph wires will be laid on to houses like water or gas.” Bell wrote to his father.

Why did Miss. Wang look so worried when we saw her.–Because she wondered ______________________.

Alice fell down and hurt ___________ just now. —We should take ___________ to the hospital.

I will carry out the plan _____ there is much difficulty ahead.