MiddleGameExercise4/solution

Global threat  

Although the lower right has been badly cut up with a ripped keima, Black's strength at the top is just too much to bear, especially since the White group is weak too. A fortiori, c is not enough either.

Local threat  

White can resolve things locally by cutting through with and but once again, Black's strength prevails and with he takes up a grand position.

Connect  

So, somewhat disappointingly perhaps, connecting at is the best option. The important cut at is still there and Black needs to deal with it.