IntroExercise4/Answer

Status?  

If black plays here, white plays and has two eyes. The stone inside white's eye can't do anything: white can capture it at any time.

Status?  

The same thing happens if black starts on the other point. Many times a group will be alive because it has two equivalent ways to live. In Japanese they say the two moves are miai.

elsewhere  

here does not even require an immediate response: white is alive even if she plays elsewhere. The two middle points are still miai for life: If black plays a, then white plays b, and vice versa.