There should have been another option. E. All of the above You are right. They are all correct.

Well. it's not gnostic present (e.g.: "Water boils at 100C."), but it does have a fact-outside-of-time quality to it which is usually the purview of the simple present. So B. They're *all* possible, as has been mentioned, but for me the descending order of "rightness" is B, C, A, D. The answer itself feels a little truncated. It would be most natural for me to say, "Well, my flight leaves at 6:30, so I should be out of here no later than 4:00." (Facts: My flight leaves at 6:30. They require me to check in one hour before. It takes an hour to drive to the airport. I need some extra time in case something goes wrong. --- All simple present.) Gee, as I write this I'm convincing that I like B, more and more. :)

Thanks Nanren, I chose B. but my solution says it is B. I wonder why.

I guess they want you to choose the one that feels rightest :) Most right? Most natural? Natural in spoken language is often the simplest that actually says the meaning.