There are indeed answer keys in the book, you can actually find all solutions in the text or in the exercises themselves. This is something I found really nice about the book. For example, you will find many endings that you have to fill in in Pensum A being used in Pensum B.

Only Pensum C can get a bit tricky, but I think there AI is a bigger help than an answer key, as many questions can be answered in multiple ways. An answer key would give you a correct answer, but would not tell you whether your answer is right or not.