I have a frozen sink strainer in a cast iron kitchen sink. The sink/strainer is about 40 years old. How can I remove the strainer? Thank you.

I have a frozen sink strainer in a cast iron kitchen sink. The sink/strainer is about 40 years old. How can I remove the strainer? Thank you.
There are several ways to remove a basket strainer,(see images) .
If the large nut's frozen you can either cut the nut with a hacksaw and peal it it off or take a hammer and chisel and see if you can get the nut to start by tapping it counterclockwise. Squirt some WD-40 on the threads first.
Good luck, Tom