For each of the following observed changes in wage rates and migration flows from the low-wage South to the high-wage North, describe one shift in conditions that, by itself, could have caused the set of changes:

a.A rise in wage rates in both South and North, and additional migration from South to North.

b.A drop in wage rates in both South and North, and additional migration from South to North.

c.A drop in the northern wage rate, a rise in the southern wage rate, and additional migration from South to North.

Please explain them in detail. Thanks :)