The correct answer using the past progressive form of the verb in the given sentence is stated below.
Past progressive tense - The past continuous tense, commonly referred to as the past progressive tense, describes an ongoing activity or situation that was occurring in the past. The past progressive tense is created by fusing the present participle of the verb with the past tense of to be (that is, was/were) ( ing word).
The correct given sentence- They had to work hard when the fire alarm stopped them.
After applying past progressive form of the verb, the sentence will be-They were working hard when the fire alarm stopped them.
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