Which of the following is the best example of an intervening opportunity that influenced migration? a. many african american citizens were heading to chicago after the civil war, but settled in border cities just north of the deep south. b. americans from the south and the east migrated to california in search of gold in the late 1800s. c. some us citizens migrated to canada to avoid the draft during the vietnam war. d. many oil workers migrated from texas to wyoming when oil was discovered near evanston.