In square of opposition which one of the following is contradictory of 'All S is P'?

In the traditional square of opposition, the contradictory of the A proposition 'All S is P' is the O proposition 'Some S is not P': the two can neither both be true nor both be false. 'No S is P' is the contrary of A (both cannot be true but both may be false), and 'Some S is P' is its subaltern, so 'Some S is not P' is the correct answer.