In the following sentence, a blank is given which must be filled with the most appropriate conjunction. Select the correct alternative out of the four given options. ______ the rain stopped, the concert had to be suspended.

Introduction / Context:
This is a sentence completion question that tests your understanding of subordinating conjunctions related to time and condition. The sentence describes a concert that could not continue during a period of rain and suggests that the suspension lasted for a certain duration. You need to choose the conjunction that best expresses this time relationship in clear and natural English.

Given Data / Assumptions:
- Sentence: ______ the rain stopped, the concert had to be suspended. - Options: Until, Unless, Till, while. - The situation is that rain forced the concert to remain suspended for a length of time.

Concept / Approach:
The structure of the sentence indicates that the suspension of the concert continued up to the time when the rain stopped. In English, we commonly express this idea with the conjunctions until or till, which show that one state or action continues up to a particular point in time. Among the given options, until is the more formal form and fits neatly at the beginning of the sentence. The conjunction unless expresses condition rather than duration, and while usually describes something happening during the time another action is in progress, not the end point of a suspension.

Step-by-Step Solution:
Step 1: Understand the meaning: The concert remained suspended for the entire period during which it was raining and only stopped being suspended when the rain stopped. Step 2: Test until: Until the rain stopped, the concert had to be suspended. This means that up to the moment when the rain ceased, the concert could not continue, which fits the intended sense. Step 3: Test unless: Unless the rain stopped, the concert had to be suspended. This sounds like a conditional rule and suggests a different structure. It would be more natural as The concert had to be suspended unless the rain stopped, and even there the emphasis shifts. Step 4: Test till: Till the rain stopped, the concert had to be suspended. This is grammatically possible and close in meaning, but until is generally preferred at the beginning of a formal sentence. Step 5: Test while: While the rain stopped, the concert had to be suspended is incorrect because while would normally refer to an ongoing action, not to the moment when the rain stops.

Verification / Alternative check:
Consider a simpler example to confirm the pattern: Until the storm passed, the match had to be stopped. The structure is identical and clearly expresses duration until a particular point in time. Replacing until with unless in such examples changes the meaning from duration to condition, which does not match the original sentence. Also, using while the storm passed feels wrong, because the word while suggests simultaneous actions, not an end point. This reasoning confirms that until is the most appropriate option.

Why Other Options Are Wrong:
Unless is wrong here because it expresses a condition (if not) rather than a time span that continues up to a specific moment. Till is a near synonym of until, but in formal contexts and at the beginning of a sentence, until is usually preferred and more standard. While is wrong because it suggests that two actions occur at the same time, whereas here the focus is on the period leading up to the stopping of the rain.

Common Pitfalls:
Students sometimes confuse unless and until, especially when translating from their first language. A helpful rule is that unless usually equals if not and introduces a condition, while until relates to time and shows the point up to which something happens. When you see a sentence that clearly describes an activity or state continuing up to a specific time, such as the rain stopped, until is usually the safest choice. Practising with similar sentences will help differentiate these conjunctions quickly in exam situations.

Final Answer:
The conjunction that best completes the sentence is Until.