When I arrived I saw the place was already ________ by two strangers in uniforms.[ ]

I don’t know what I _______ without the suitcase you lent me.—Glad to have been of some help to you.

It is important _________.

Don’t worry, sir. I’m sure I can run to catch up with them.

It is pretty well understood ________ controls the flow of carbon dioxide in and out the atmosphere today.