If you don't know a person's last name, use 'ma'am' or 'sir'.
如果你不知道一人的姓, 你应称呼他为 'ma'am' 或 'sir' 。
I had several photographs of Clare Flower, and I picked out one interesting photograph of Clare aged twelve in an art class at school.
我有几张克莱尔·弗劳尔的照片,挑出了一张克莱尔12岁时在学校假期艺术课上拍的很有趣的照片。"