I really enjoyed reading your question. The explanation was clear and almost scientifically accurate but you add a long presentation instead of a short explanation and short animation that made it more interesting and could wake up pupil curiosity.
About the multi choice question:
It's a calculation question that need understanding the relations between variables and parameters. its a good question wrote in a correct way.
The answers: I think that the answers should built the way that if students use wrong parameters or variables will got one of the answers that you add. so i think that the answers built in a way the student can try until he got the correct answer.
Why do you think that the student will chose the wrong answer? you should add a note after the wrong answer that explain why you think that the student got and chose this answer and try again using your note.
Correct answer: you add the correct method the student should answer the question and it's good to see that we answer it in a correct method.