21. While being assisted by an icebreaker on Lake Superior, you receive a single letter code instructing you to slow your vessel. Which signal could you use to signify that you are complying with the request?

To acknowledge an icebreaker's instruction to slow, use four whistle blasts in the pattern of one short, two long, and one short, which corresponds to the letter «P» in the International Code of Signals and signifies compliance.