If a^(2/3) = b^(2/3) for a ≠ 0 and b ≠ 0, then which of the following statements must be true? Indicate all possible values.

a. a/b = 1

b. a/b = -1

c. (a - b)² = 1

d. a = 2/3

e. a² = b²

f. √a = √b