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.