Visual Studio 2008在解决方案中切换活动项目

I don't know how it happened, but for about a week now Visual Studio keeps switching the active project everytime I move between files (of different projects) in the same solution. Of course when I press F5 to start debugging or Ctrl+F5 to run the tests, it tells me that it can't start because the class library can't be started directly... It switched the active project again.I don't know how it happened, but for about a w