Which of the following can be a right way of doing so$?$

A$.$ R$1=1,$ if East Asia, and $0,$ otherwise; $R2=1,$ if Africa, and $0$ otherwise.

B$.$ R$1=1,$ if East Asia, and $0,$ otherwise; $R2=1,$ if Africa, and $0$ otherwise; $R3=1,$ if Latin America, and $0$ otherwise.

C$.$ R$1=0,$ if East Asia, and $1,$ otherwise; $R2=1,$ if Africa, and $0$ otherwise.

D$.$

$R=1,if$ East Asia;

$=2,if$ Africa, and

$=3,if$ Latin America.

B$)$ Followed from last question, which region is the base case?

A$.$ Africa

B$.$ East Asia

C$.$ there is no base case.

D$.$ Latin America