This question checks your understanding of conditional conjunctions. The sentence talks about a situation that will succeed only on one condition—if everyone cooperates. Details Sentence: The plan will work only if everyone cooperates. Rule: Use if to express a condition that must be met for something to happen. Examples: You will pass if you study. The machine will start if you press the button. Why not others? unless = means if not (opposite meaning) although = shows contrast despite = used be