The words like who – which – that may be followed by singular Verb and Helping Verb or plural Verb and Helping Verb.
was /wereMr.Chips's favourite.
The Verb or Helping Verb after who – which – that are used according to the word written immediately before these words. The word before who – which – that may be singular or plural. So, use the Verb or H.V accordingly.
Examples:
(i) He is one of the bright boys who has/have always done the school work without effort and diligence.
(ii) Inspector French was one of the books which
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