The correct option for the sentence "What a relief that the strike .... when it did!" is C. ended. Let's break down why this is the best choice and why the other options are less suitable. Step-by-Step Explanation

1. Understanding the Context: The sentence expresses a feeling of relief regarding the timing of the strike's conclusion. The phrase "when it did" indicates that the event (the end of the strike) is being referred to in the past.
2. Analyzing the Options:
3. A. would end: This option uses the modal verb "would," which suggests a future or conditional action. It implies that the end of the strike was expected but does not fit the context of expressing relief about something that has already happened. Therefore, this option is incorrect.
4. B. was ending: This option uses the past continuous tense, which indicates an ongoing action in the past. It suggests that the strike was in the process of ending at a certain time, but it does not convey the finality or completion that the sentence requires. Thus, this option is also incorrect.
5. C. ended: This option uses the simple past tense, which is appropriate here because it indicates that the strike has definitively concluded at a specific time in the past. The phrase "when it did" aligns perfectly with this tense, making it the most suitable choice. It conveys a sense of relief that the strike ended at a particular moment, which is the intended meaning of the sentence.
6. D. has ended: This option uses the present perfect tense, which is typically used to describe actions that have relevance to the present moment or that occurred at an unspecified time before now. However, the context of the sentence is focused on a specific past event, making this option less suitable than option C.

Summary of Why C is Correct

• C. ended is the best choice because it accurately reflects the completed action of the strike concluding in the past, aligning with the expression of relief about that specific timing.

Summary of Why Other Options are Incorrect

• A. would end: Suggests a future or conditional action, not suitable for expressing past relief.
• B. was ending: Indicates an ongoing action rather than a completed one, which does not fit the context.
• D. has ended: Implies relevance to the present, which is not appropriate for the specific past event being discussed.

Revision Summary

• The correct answer is C. ended because it indicates a completed action in the past.
• The context of relief requires a past tense that shows finality, which C provides.
• Options A, B, and D do not fit the context of the sentence due to their tense and meaning.
• Understanding verb tenses is crucial for selecting the correct option in fill-in-the-blank questions.