Hi ginnu,
I want to know crossing border from Canada to usa by road or by air which one is better,
----- Both are better
&
What did they ask while crossing the border.
---- While you enter US from Canada they ask when did you go to Canada? Have you bought anything? What you do in US. They just check the passport and Visa sticker and I-94.
If you go by Air to Canada the airline does not ask you to surrender the I-94 departure to Canada is considered domestic flight, when you come back to US from Canada by Air you face the US immigration at Airport in Canada before boarding a plane for US, show the passport Visa and I-94