Tsumego Set #4

Elementary:At first I was going to say B6...but from the way I see it, that would play out B6b, D6w...then watever, ruining the possibilities of 2 eyes. I guess it would be C7, opening up possibilities to cap the upper left for 1 eye and then the second eye would be in the D row.
Int.: At first I leaned on B5, but A6 getting taken would ruin any abilities to put white in atari from what I see. The best tihng I see (which is probably too safe, is D8, trying to trap white. The other possibility I see is D5, protecting C6/D6 making it harder for white to get eyes in there and closing off the area.

Meh, long winded and still I didn't have a definite answer.

Elementary: C7 is a good first move, then white follows with D6. What do you play next?

Intermediate: Your first intuition was correct, B5 is the vital point splitting the 2 groups. Here's the full sequence: bB5, wA5, bC5, wA7, bD5, wA6, bD8. The entire group is dead. If white responds to bB5 with wC5, black can play at D5. Now if white caps with A5, black can play a snapback move at B5 capturing the lower group. If you start the sequence with bD8, white plays at B5, solidifying his group with ample space to make 2 eyes (in ko). bD8, wB5, bA7, wA8, placing the group in a ko fight (not as good as an outright kill).

The best I see is go B6, white goes D7, taking 2 stones, black goes A7, making 2 eyes. I am confused if there is something better. I had wanted to play D7 next but I see that leads to all the stones being taken (whiteD5.