Returns a value from a list, given an index number.
CHOOSE(index; value1; value2; ... value30)
CHOOSE(1; "dog";"cat")
CHOOSE(2; "dog"; 65.4)
CHOOSE(3; "dog"; 65.4; B5)
SUM(CHOOSE(1; B4:B5; B6))
CHOOSE(2;SUM(B1:C1);SQRT(9))