一、词汇和语法（本大题共 20 小题，每小题 1 分，共 20 分） 根据句子的意思选择一个正确的答案，错选、多选或未选均无分。 1. It‘s______ by coach from Melbourne to Sydney. A. a nine hour‘s journey B. a nine hour journey C. a nine hours‘ journey D. a nine-hours journey

Definition A tree is a collection of nodes. The collection can be empty: otherwise, a tree consists of (1) a distinguished node r, called the root; (2)and zero or more nonempty(sub)trees T1,., Tk, each of whose roots are connected by a directed edge from r Note:

Random walks 1 Random walks a drunkard stumbles out of a bar staggers one step to the right, with a canal lies y steps to his right. Thi I equal p second, he either staggers one step to the left or probability. His home lies r steps to his left, and everal natural questions, including 1. What is the probability that the drunkard arrives safely at home instead of falling into the canal? 2. What is the expected duration of his journey however it ends? The drunkard's meandering path is called a random walk. Random walks are an im- portant subject, because they can model such a wide array of phenomena. For example