In each of the following questions, three sentences are given with an bold phrase or words. Choose the single phrasal verb from the options that can grammatically and contextually replace all of the bold phrases or words across all three sentences without changing their core logical meaning. If the sentences are correct as they stand, choose "No improvement required". 1.      When the lead vocalist unexpectedly fell ill hours before the concert, an understudy had to substitute for her on the main stage. 2.      The newly appointed forensic accountant was tasked to investigate the intricate financial discrepancies buried deep within the subsidiary's ledger. 3.      ​In highly volatile geopolitical environments, regional skirmishes can easily represent a much broader ideological proxy war.