Why Did John Porter and Other Sociologists Modify Their Views

Why did John Porter and other sociologists modify their views on the vertical mosaic by the 1970s?

A) because economic differences among ethnic groups diminished
B) because Canadian-born people began to do less well economically than recent immigrants
C) because the structure of mobility opportunities in Canada was reduced significantly in the decades after World War II
D) because immigrants began to arrive in Canada with higher levels of education