(915, #29) Determine whether or not the following set is (a) open, (b) connected, and (c) simply-connected.

Solution: The region D is the first quadrant.

1. D is open, around every point in D we can put a disk that lies in D.
2. D is connected, any two points in D can be joined by a path that lies in D.
3. D is simply connected, it's connected and has no holes.

By C.A.H.