How to represent selected items in a list?

Imagine a list (A) of items i.e. rectangles with fixed width but flexible height each of which contains a string. What I want the user to be able to do is to select multiple items from this list and then based on a (non-fixed) set of rules gets a list (B) of recommendations (that looks like the items in the list A, but with possible different content) of which she can chose which ones to import to list A.

(it's quite important for the user to see on which items the recommendations are based on)

Question: How to represent which items (from A) were selected?

  1. simply put a checkbox on the items?
  2. clone them to a separate list (which is possibly next to list A) above|below which the recommendations appear?
  3. checkbox, but anchor the selected ones to the visible area, so when she would scroll past them they stick to the top or bottom (but this would put a limit on the number of selectable items)