Answer:
A. Securely
B. "...no way in the world can you get rid of the nasty hard thing..."
Explanation:
A. The word fast in this context means securely. For example, if I held my money fast it would mean that I clutched it tightly to make sure nothing could happen to it.
B. This quotation shows that the harness was held securely on the horse, since he couldn't get it off.