The following code block is a $\_\_$str$\_\_()$ method for a Stack class used to implement a stack data structure in Python. What code belongs in the blank if the desired behavior for this method is to return a string that can be used to display the elements in a stack from top to bottom?

$$

def $\_\_$str$\_\_($self$)$:

$$ string $=$ "Top to bottom: $"$

$$ traverser $=$ self.top

$\_\_\_\_\_$

$$ return string

a$.$

while traverser $==$ None:

$$string $+=$ str$($traverser$.$data$)+""$

$$traverser $=$ traverser.next

b$.$

while traverser $!=$ None:

$$string $+=$ str$($traverser$.$data$)+""$

$$traverser $=$ traverser.next

c$.$

print$($self$.$data$)$

d$.$

for i in traverser:

$$string $+=$ str$($traverser$.$data$)+""$

$$traverser $=$ traverser.next