父母去年来过，住了半年，现在在国内，最近签证要过期了。一定要过期之后才能重新申请吗？
-Not really. If they will come to US after their current B2 visa expires, they can apply for the new B2 visa before the current B2 visa expires (no law limits that).
这次想去加拿大和墨西哥玩玩，可以在中国直接申请这些国家的签证吗？
-Yes.
如果来美国再申请呢？
-Up to them. But they do not need a visa to enter Mexico if they go to Mexico from US directly (the Mexico Customs officer will give the tourist visa to them at the entry point). But they need to get the Canadian visa to enter Canada.
