What do the letters “DOS” ______？—Disk Operating System.

The _______, I think, he drew from his simpleexperiment is not scientific.

Did Tom come back late last night—No. It was not yet 8 o’clock ____ he arrived home.

The inspector ____ his fingers over the sheet of newspaper. There were some marks on the surface of the paper. [ ]

________they visited the Science Museum，they showed great interest in everything they saw