Woman: John, are you doing research for Professor Williams this semester？ Man: Actually, I

Woman: John, are you doing research for Professor Williams this semester？ Man: Actually, Im working as his teaching assistant. Question: What does the man mean？

A．He needs another job as research assistant.

B．He asked Professor Williams for assistance.

C．He assists Professor Williams with his teaching.

D．He is doing research with Professor Williams.

请帮忙给出正确答案和分析，谢谢！