I-485 + more info & evidences

Just received a letter from INS asking for more info.and evidences.
Here is what they said:

Please submit evidence that you are not likely to become a public charge. Evidence my include:
a) an original letter of employment(stating prospect for continued employment and salary), or
b) a Form I-134 completed by your financial sponsor for which your sponsor must comply with the instructions of the form, or
c) other evidence(bank letters, etc.)

Questions:
1. what is 'public charge'?
2. Should I send everything I can provide, or just the easiest.
3. If I can send just one evidence, which one is the best to send.

Thank you!!!


EB-2
Vermont Center
I-140 Rd 03-14-2004
I-485 Rd 03-14-2004

Finger Print 11-01-2004
I-140 Approved 11-05-2004