Line 3 follows from lines 1 and 2 by Modus Ponens P Q R S

Line 3 follows from lines 1 and 2 by Modus Ponens (P > Q) > (R > S) R S

Solution

1) (P > Q) > (R > S)

2) R

3) S

Now 1) A-> B

2) C

3) D

Now A is clearly P>Q

B Is R>S

But A does not appear on the list again

hence by Modes ponens, the statement is false