２００１ 年还是F１学生，去加拿大开会，记得那是好象是持I２０和I９４在３０天内就可以回美不需要再签证。不知怎样证明，需要什么样的材料？有经验的大侠们请出出主意，多谢!
Here is the RFE copied from the letter:
On your From I-485, you state you last entry into the united states wase July 22, 2001. However, it is noted that in your passport you have canadian immigration stamp dated July 5, 2002. Submit evidence of you last entry into the united States prior to filing your Form I485.
If a Form I-94 is not available for you last entry, you must explain what became of the form and submit a copy of the personal information pases from your passport and those pages endorsed by immigration officials establishing you admissions or parole into the United States. If you were not issued a Form I-94, please provide a detailed explanation inclding the length and purpose of your trip and provide any documentary evedence to support you statements.
Because the most current immigration stamp in you passport is from Canada, it appears you were not in the US at the time you filed you I-485. Submit evidence that you were physically present in the US on August 1, 2007.