X visa application for Nigerians

If you have all the required documents for China student visa application, it is easy for Nigerian passport to get a student X visa, such as the admission letter, JW201 or JW202

I'm puzzled by your second question, X visa holders should change it into a residence permit within 30 days after entering China, the residence permit should be renewed every year. I didn't hear the extension of X visa. As I know, when the foreign students will end the study in China, the local exit-entry administrative department will issue a 10 days exit visa to them for packing and leaving China on time.