This question tests the use of conjunctions to connect contrasting ideas. The sentence presents a contrast; the project was difficult, yet it was completed on time. 'But' is the conjunction that expresses this contrasting relationship between the two clauses. 'So' indicates a result, 'because' indicates a reason, and 'therefore' indicates a conclusion. None of these fit the context. Therefore, Option B is correct. * **Core Concept:** Use of conjunctions to express contrast * **Solving Strategy:** Identify the conjunction that best connects contrasting ideas. * **Option Analysis:** * **A:** Incorrect. 'So' indicates a result, not a contrast. * **B:** Correct. 'But' shows a contrast between the difficulty of the project and finishing on time * **C:** Incorrect. 'Because' explains a reason, not a contrast * **D:** Incorrect. 'Therefore' indicates a result, not a contrast * **Easy Mistakes:** Confusing 'but' with 'so', 'because', or 'therefore'.