Why have some historians reject the labels bc and ad as labels for periods in world history

Well, if you didn't know B.C stands for Before Christ which is using mainly by Christians and A.D stands for Anno Diomini or the Year of the Lord which again, is used by Christians. They prefer to use B.C.E (Before Common Era) and CE (Common Era) as it doesn't show anything to do with religion. This is what I think.

the second significant cause of the great migration was the desire of black southerners to escape segregation, known euphemistically as jim crow. rural african american southerners believed that segregation - and racism and prejudice against blacks - was significantly less intense in the north.

answer: the risk-return relationship. generally, the higher the potential return of an investment, the higher the risk. there is no guarantee that you will actually get a higher return by accepting more risk. diversification enables you to reduce the risk of your portfolio without sacrificing potential returns.

explanation: