根据句意，用括号内所给词的适当形式填空。
91. After so many ________ (year) practice, Li Na is a well-known tennis player.
92. We’ll have a 
surprise party for my mother’s ________ (forty) birthday next Saturday.
93. The higher the mountain is, the ________ (thin) the air is.
94. To tell the ________ (true), I don’t like the drinks in that café.
95. It’s a (an) ________ (usual) experience, few people have chances to do it.