From the first set of options, it would either be have or had, depending on the intended meaning. 'Has' is third person singular, and so cannot work with the plural subject 'decades'. 'Have' denotes that you are speaking of the present, while 'had' denotes that you are speaking of the past.
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Jin_Hso 'had' is used for both singular and plural subjects when talking about the past?That's generally true, but it's a bit of an oversimplification.