obvious answer is that the structure of (lb) involves a wh-word that moves from the complement of the matrix verb and that the second question is therefore a separate CP, as in (5).
Examples like (lb) and (4) raise the question, where does the wh-word come from? The obvious answer is that the structure of (lb) involves a wh-word that moves from the complement of the matrix verb and that the second question is therefore a separate CP, as in (5).