回複：B1/B2 往返簽證可以從第三國入境美國麽？

Why will you ever raise this kind of question?

First of all, if it doesn't work as what you thought, how come so many Chinese working or studying in the U.S. still go to Canada for visa and then go back to China and then come back directly to U.S.?

Secondly, if it doesn't work as what you thought, how come the U.S. embassy didn't add any anotation to the visa or visa application info?

Use your brain. You should get a clear answer.