Android proguard, keep inner class

My android program has a class A, which has two static inner class. They are found to be stripped from .dex after applying proguard.

public class A{

  ...
  static class B{
    ...
  }

  static class C{
    ...
  }
}

I have put the following lines in proguard.flags, but seem no luck.

-keep class com.xxx.A
-keep class com.xxx.A$*

Any hint?