When diving in water to work, pearl divers ..... use Oxygen cylinders. must Have to
The model answer here is:'have to'. The author sees that it is an external obligation as the pearl divers have no choice but to use oxygen cylinders.
Yes, "have to" is a good model answer if we assume that that is the desired meaning, and it seems likely that that would be the desired meaning if the sentence were actually spoken in the world outside Egyptian grammar quizzes.
That said, it is worth considering what the sentence would mean with "must." The sentence "When diving in water to work, pearl divers must use oxygen cylinders" reads as a rule for pear divers to follow. The sentence could be written on a sign.
As to pearl divers having "no choice but to use oxygen cylinders," that is probably true for most pearl divers. However, the protagonist of the novel The Pearl, by John Steinbeck, is a pearl diver who holds his breath when diving for pearls.
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