② An algebraic expression has only one value. No! For example, the algebraic expression x can take any real number.

③ When the letters in the same algebraic expression take different values, the values of the algebraic expression must be different. No! For example, in the algebraic expression X 2, when x= 1 or-1, the values of algebraic expressions are all 1.

④ When the letters in the same algebraic expression take different values, the values of algebraic expressions are not necessarily the same. Yes, for the same reason.

So there is only one right one, choose B.