回複：Question to 6873, thanks in advance

When you drive from the US to Canada, you don't go through the US immigration but Canadian. You may surrender your I-94 card to Canadians. However, they may not take it. It will depend on the individual officer. When you drive back from Canada to the US, you will not go through Canadian agency but the US. If you surrender your I-94 on the way into Canada, you will get a new I-94 when going through the US agency provided that your visa is still valid. The length of stay is determined by the individual officer.