i have run it already and know the answer but i want someone who can tell me more about how to trace this code.

I need help with this code. i have run it already and know the answer but i want someone who can tell me more about how to trace this code. I can trace it from the beginning but get confuse at the end. Please you explanation should be well understood.

def justOnce(aString):

rtn = []

initials = []

words = aString.split()

for word in words:

initials.append(word[0])

for letter in initials:

if initials.count(letter) == 1:

rtn.append(letter)

return rtn

s = 'Seventy six trombones led the big parade'

print(justOnce(s))

a. ['S', 's', 'l', 't', 'b', 'p']

b. ['Seventy', 'six', 'led', 'big', 'parade']

c. ['s', 'l', 'b', 'p'] d. ['S', 's', 'l', 'b', 'p']

e. None of the above