You chose that the ions move out because the outer environment is more conducive to the sodium ions. This is essentially stating that sodiums ions like the outside. Ions neither like nor dislike any place. The move as a result of the basic physical forces acting on them. For example, if concentration is the sole factor being considered, the ions simply make random movements and, thus, since more are outside that might move in than inside to move out, more ions on average will move in. Examine Figure 1 and 2 from earlier in the tutorial to check your answers.
Figures 1 and 2 reshown here to illustrate diffusion.