Help the exhausted astronaut update his final log before returning to Earth by choosing the correct verb phrase.

By the time the rescue shuttle finally docks on Tuesday, I _______ at this blinking control panel for exactly 400 days.

The correct answer is will have been sitting.

We use the Future Perfect Continuous (will have been + V-ing) to emphasize the duration of an ongoing action leading up to a specific point in the future.

"Will be sitting" (Future Continuous) is incorrect because it doesn't account for the duration ("for exactly 400 days"). "Have been sitting" (Present Perfect Continuous) doesn't work because the endpoint ("By the time the rescue shuttle docks") is in the future.