The given question is incomplete.The options of the given question are:
A. Great Migration; Rocky Mountains
B. Great Migration; cities in the North
C. Civil War; houses
D. Civil War; California
Answer: Option B
Step-by-step explanation:
During the great migration, many people moved into the cities in the north.This is because the north is very stable part of the earth to live in.
The great migration can be defined as the time when a large number of people migrated towards the north to live there.
The cities in the north have better facilities and have better environment to live in.
Hence, the correct option is B.