13. Five friends go to the cinema to watch a movie, and their seats are in a row from left to right, numbered 1 to 5. During the movie, Ada goes to the lobby to buy some popcorn. When she returns, she finds that Bea has moved two seats to the right, Cissy has moved one seat to the left, Dee and Idi have swapped seats, and only the leftmost seat is left for her. Therefore, Ada initially sat in seat ( ) number.
(A) 1
(B) 2
(C) 3
(D) 4
(E) 5
13. B.

Let $(A, B, C, D, E)$ be the initial seat numbers of five people. Then
$$
A+B+C+D+E=1+2+3+4+5 \text {. }
$$

Assuming the positive direction is to the right, if Ada moves $x$ seats, then
$$
\begin{array}{l}
A+x+B+2+C-1+E+D \\
=1+2+3+4+5 \\
\Rightarrow x=-1 .
\end{array}
$$

Thus, after moving 1 seat to the left, Ada sits in seat 1. Therefore, Ada initially sat in seat 2.