How to display Serial Number for items in Table with Expanded Rows?

I have a generic table with a list of items and columns. Some items can be expanded via an 'expand row' icon located at the leftmost side of the table, which displays a list of subitems underneath the item row.

I need to show S/N or serial numbers for all the items, including subitems

I have thought of 3 ways of displaying S/N:

A. Items & Subitems have their own list of S/N
Items numbered 1,2,3... Subitems numbered 1,2,3...

B. Subitems S/N tied to Item S/N
Items numbered 1,2,3... Subitems numbered 1.1, 1.2, 1.3...

C. Items & Subitems share a single list of S/N
Items numbered 1,5,6,7,9... Subitems numbered 2,3,4,8 etc...

Which method is most user-friendly?

B seems the neatest, but I'd like to keep the S/N as a single integer if possible (like C). The problem with C is that the S/N skips numbers when the rows are not expanded, which seems unintuitive.