Excel Formula for automated WBS Numbering

Good Evening All

Is there a formula to automate a WBS number based on levels, as seen in the attached spreadsheet that takes into account blank rows as well?

so if I select the level via drop down on column B, column c is populated automatically and column d (nice to have) automatically idents the respective task?

I dont want to use a macro, was hoping simple formula's could do the trick in column c / d?

It should also take into account blank rows and continue sequential numbering from above.

Thank you in advance...