Final answer:
The expression equivalent to 's - t' is 's - t' itself, as it represents the difference between the two variables, s and t.
Step-by-step explanation:
The expression equivalent to s - t is simply s - t itself because it represents the difference between the two variables s and t. Options A and B involve changing the sign of s which would not be equivalent, and option C adds t instead of subtracting it, while option D represents only the variable s without t. Therefore, none of the other options are equivalent to the original expression s - t.