Assume that there are only three possible levels of education: "low", "middle" and "high". Among Norwegian immigrants in the US, 50% have "low" education, 50% have "middle" education and 0 % have "high" education. Among Norwegian stayers, 100% have "high" education. The Net Difference Index comparing the education of Norwegian immigrants in the US to the education of Norwegian stayers is equal to:
a. -1
b. -0.5
c. 0
d. 0.5
e. 1