Boolean difference problem

rtc1.2.3dm (1.5 MB)
I am new to rhino and i designed this simple ring but when i choose boolean difference it does not get rid of the extrusion for the ring size, i have placed the file above if anyone can give me some advice i would greatly appreciate it

I assume you used BooleanDifference. Was the option DeleteInput=Yes or DeleteInput=No? The option is changed in the command line. It appears that it was DeleteInput=No.
https://docs.mcneel.com/rhino/7/help/en-us/index.htm#commands/booleandifference.htm