One Recently Published Theory as to Why Unionization Rates Are

One recently published theory as to why unionization rates are much higher in Canada than in the U.S. is that:

A) the demographic traits of the labour force in Canada are those that are associated with higher rates of unionization.
B) changes in the U.S. legal regime since the end of World War II related to unions have been less favourable to unionization.
C) Canadians' social and political values are relatively more collectivist and social democratic in nature, and hence less individualistic and free-market oriented.
D) the industrial composition of the labour force in Canada is weighted toward those associated with higher rates of unionization.
E) Canadians' social and political values are relatively less collectivist and social democratic in nature, and hence more individualistic and free-market oriented.