Though congress passed legislation to restrict immigration in the early 1920s, one border was left relatively open. which border was this, and why? a)the canadian border was left open because canada and the united states had recently signed a treaty to restore peace in the north american region. b)the east coast was left open because of business agreements with eastern european countries, which brought money to the united states. c)the mexican border was left open because labor coming from mexico was particularly cheap, and many agricultural workers were needed. d)the west coast was left open because asian immigrants had recently been allowed to enter the united states following a long period of exclusion.