def replaceWithQuestionMarks(lines):
    # (assumes each line in lines is the same length)

    newLine = [] # this will contain the characters/question marks

    # loop over each index in a line:
    for i in range(len(lines[0])):
        charactersInAllLines = []

        # loop over each line, populating charactersInAllLines with the ith character in the line
        for line in lines:
            charactersInAllLines.append(line[i])

        # convert to a set to get rid of duplicate characters, check if there's only 1 unique character
        if len(set(charactersInAllLines)) == 1:
            newLine.append(charactersInAllLines[0]) # add the character to the new line
        else:
            newLine.append('?') # append a question mark

    return ''.join(newLine) # convert the list of strings into a single string


print(replaceWithQuestionMarks(['121212123434AA6AAA', '121212222234AAAAAB', 'D21212122434AAAAAF']))