Graded Assignment CT Question 21 and 18

[Pranjal Sarkar]

Hi,

In question number 21, why should 

Serial number of each item bought in the “Shopping Bills” dataset

be an integer?

We don't know how many items are there. So basically it should be a list.

Okay, even if I consider that the serial numbers are integers, then why should subject marks be a list(subject marks in question 18)??

Please explain what misconception has gripped me.

Thanks 

Pranjal Sarkar