Peter has sent 100 million satoshis to Carol to purchase a diamond ring for his fiance. He showed to Carol a screenshot of his transaction that has received a 1-block confirmation and Carol proceeded to arrange for delivery of the ring. Half an hour later, Peter realised that his transaction of 100 million satoshis to Carol was pending. He quickly called up Carol, who stopped the delivery man from picking up the parcel. (a) Analyse this incident and recommend the next step for Peter and Carol. (b) Appraise the security and usability of Bitcoin as a payment system. (10 marks) (30 marks)