What type of visa should I apply for ?

Sir, Madam, I am a 3rd year student of the American Business School, a Nigerian citizen and resident in France. I have an invitation from a company in Beijing to undergo an internship for 3 months starting from 01/16/2011 to 31/08/2011.
I wish to know what type of visa do I require and if I could ask for a double entry within 6 months? I have also been to China last October 2010 with an "F" visa for 6 days. Thank you for your prompt response.