Traditional Square of Opposition

Contrary = At least one must be false (not both true, but both could be false)                                        

Subcontrary = At least one must be true (not both false, but both could be true)                                 

Contradictory = Opposite truth value                                                                                                           

Subalternation = Truth flows downward; falsity flows upward.

Question: Using the Traditional Square of Opposition above, if "All X are Y" is true, what would the truth value of "Some X are not Y" be?

Select one:
 a. False
 b. True
 c. Undetermined