Which experiment method is important for controlling bias ?

The answer is C. Because if you let them pick their group it will be their opinion so it will be biased.And the other two wouldnt help at all.Therefore C is the answer Good luck :)

what are the choices? the multiple choices?

i will answer in the comments after the choices have been said.

    racial conflict and discrimination has existed ever since man learned to kill with rocks and bones. these conflicts arose from disputes between territory, power, wealth, religion, or simple difference. some of the most infamous racial disputes include slavery, the holocaust, and the civil rights movement.

    these listed disputes often resulted in casualties from all sides. riots, wars, imprisonment, and lynching are all commonly associated with racial conflict on severe levels.

      however, racism is an improving issue in recent decades. people are being more accepting and welcoming of diverse populations. though, the world isn't perfect and it will take much more time, if ever, until the world is rid of racism.