是介词。例句：They arrived late at the airport.他们晚到了机场。I'll be at home all morning.我一上午都在家。
at的用法
at只能做介词，意味着它后面必须跟一个名词或动名词。
at可以用在时间几点几分前，比如at 7:00，at 10:15
at也可以在地点前，at home，at school，at the top of the mountain，at the foot of the mountain.
at还可以用在一个明确的目标点前，如point at him指着他，throw at him扔向他，shout at him对着他嚷嚷，laugh at him嘲笑它，smile at him，对着他微笑，surprised at the news对这个消息感到吃惊。