Then suddenly, despite all their ______ (assurance/ assurances), they broke the agreement and signed a deal with a rival company.
I’m not sure which form of the word ‘assurance’ should be taken in this sentence. I think the word assurance itself means ‘promise’ here, but not ‘ confidence’ , and according to the dictionary it’s a countable noun. But our teacher provides us with the answer ‘assurance’ the single one. Is the answer wrong?