BG5EGC Before taking the C-level exam, you must verify your license. If it's a paper license, you need to bring the physical copy; if it's an electronic license, you need to print it out. Furthermore, you must be able to find the license that proves you have held a B-class radio station license for 18 months or more on the local government website at the time of the exam. If the provided license is less than 18 months, you must obtain a related proof from the issuing authority. Generally, even without a proof, you can still take the exam, but you must submit this proof before the results are released. If you cannot submit it, the results will be invalid.
Personally, my paper license hasn't been valid for more than 18 months. I went to the issuing authority to retrieve my original B-class license.