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' 。
The World Trade Center, an office building for over 1200 firms employing about 50,000 people, is even higher at 411 metres.
世界贸易中心要高得多,有411米。它是一座办公大楼,楼里有1,200多家公司,大约有5万雇员。