Nested lists can get very complex very quickly, and trying to go "down every level" in a nested

list is both time$-$consuming and hard to keep track of$.$ This is why when we're thinking

recursively, we rarely go down more than one level in a nested list.

To prepare for this idea, review from the readings the definition of sub$-$nested$-$list, and then

answer the following question:

How many sub$-$nested$-$lists does $[[15,[105,13]],[0,1,2,3],4,\left[\begin{array}{c}?\end{array}\right]]$ have?

$8$

$4$

$6$

$1$

$13$