________ at the building site, the workers at once threw themselves into the work.A.At arrivingB.On their reachingC.As soon as they gotD.On their arrival
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D解析介词on可表示“一……就……”，与as soon as意思相同，on后接动名词或arrival，return；at可表示“一看见/听到”，如at the news，at the sight of．