Fill in the blank with an appropriate modal which demonstrates a past possibility:
People ______ think that the earth was flat.
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We use modal verbs to show if we believe something is certain, probable or possible (or not).
From the given options, option B - 'used to' is correct as it is used to say that something happened continuously or frequently during a period in the past. 'Used to' thus demonstrates a past possibility here.
The incorrect options are:
Might - it is used to show that something is possible in the future, but not certain
Will - is used to talk about the future – to say what we believe will happen
Would - is used as a past tense of will and to talk about hypotheses. Whereas the given sentence is a fact and not a hypotheses