Thanks, Behavioral. As this is my first year in the program and I still have some time to think about my application, your advice help me a lot to plan my next step.
I took Calc I-III, LA and probability theory when I was an undergraduate. Although I got nearly full marks in those three, I don't think it's enough. I am seriously considering a economic master now. In addition, I am applying an exchange to Tilburg University (in the Netherlands), and ideally, I may take a lot of courses in economics major there. Hope this could help. I really I don't want to spend another 2 years in a master program. :(