Words A and B need to be interchanged. In the place of word A, a plural noun is required as it is preceded by the word 'three' (denoting plurality) and in the place of word B, an auxiliary verb is required before the main verb 'braved'. Accordingly, word A is an auxiliary verb and word B is a plural noun so they can be interchanged. The other two words are appropriate in their respective positions.
The correct sentence is: In the past three decades, Bangladesh has braved more than 200 natural disasters, by some estimates.
Hence, the correct answer is option (a).
Note:
Auxiliary verb: A verb that gives grammatical information that is not given by the main verb of a sentence. It supports the main verb. Auxiliary verbs help to form the various tenses, moods, and voices of other verbs. The principal ones are be, do, and have. Example: She will sing a song tomorrow. Here, 'will' is the auxiliary verb that supports the main verb 'sing'.
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