He was so deep in thought, thinking of the problem _________he knocked into the tree without noticing it.
Students should be encouraged to use _____ Internet as _____ resource.
His sister left home in 1998, and_______ ever since.
Sam _____ some knowledge of the computer just by watching others working on it.
-----Have you been wasting time on computer games again? ----- _______. I’ve been studying a lot and I need a break.
When I got on the bus, I_____ I had left my wallet at home.