13. When Clara calculated the total score of her test results in each subject, she unconsciously swapped the tens and units digits of a certain subject's score. Then the score that is most likely to be the difference between the incorrect total score and the correct total score is ( ).
(A) 45
(B) 46
(C) 47
(D) 48
(E) 49
13. A.

Let the correct total score be $10 a+b$. Then the incorrect total score is $10 b+a$.
Notice that, the difference between the two is
$$
10 a+b-(10 b+a)=9(a-b) \equiv 0(\bmod 9) \text {. }
$$

Since only option $\mathrm{A}$ fits, the most likely score difference is 45.