Two Quiz.questions lists merging together - how to stop it?

I wrote the following code myself for the Dates and Times in Python course, before watching the video "The Quiz Class". I made some minor edits while watching, as I saw some ways to complete the program that were better than how I'd programmed it myself. I should also note that I customized my code to have a variable number of questions - see the following:

class Quiz:
    questions = []
    answers = []
    def __init__(self, num_of_questions):
        question_types = (Add, Multiply)
        for _ in range (num_of_questions): 
            self.questions.append(random.choice(question_types)(random.randint(1,10), random.randint(1,10)))

While I was messing around and testing my code, I entered the following into the console:

import quiz

ab = quiz.Quiz(5)
for x in ab.questions:
    print(x.text)

4 + 2
6 * 4
10 * 10
3 + 4
2 * 7

So far, this is correct. However, I then made another quiz instance...

ac = quiz.Quiz(5)
for x in ac.questions:
    print(x.text)

4 + 2
6 * 4
10 * 10
3 + 4
2 * 7
6 + 4
4 * 7
7 + 6
3 + 9
6 * 6

Repeating the "for x in ab.questions" code as directly above, which is ab and ac seemingly merged together. I have a vague familiarity with Assembly code (which deals directly with memory in a way Python does not), which was enough to give me at least a general idea of what is happening here. As far as I can tell, ab.questions and ac.questions are taking up adjacent slots in memory, and the processor can't tell where ab ends and ac starts. As such I end up with some amalgamation of the two. Were I writing this in Assembly I could do an alignment, use a filler variable, or something else of that nature - here in Python, however, I have no idea how to fix this problem. Does anyone have a suggestion?