刀具材料应具备的性能有哪些？
参考答案：
1、较高的硬度；2、足够的强度和韧性；3、较好的耐磨性；4、较高的耐热性；5、较好的工艺性。
已知a，b都是实数，那么“a2＞b2”是“a＞b”的[ ]
A．充分而不必要条件
B．必要而不充分条件
C．充分必要条件
D．既不充分也不必要条件
语法填空。
People 1_____ (live) in different countries made different kinds of words. Today there are about fifteen
hundred 2_____ in the world. Each contains many thousands of words. A very large dictionary, for example,
contains four 3_____ five hundred thousand words. But we do not need 4_____ these. To read short stories
you need to know only about two thousand words. 5_____ you leave school, you will learn only one thousand
or more.
The words you know are called your vocabulary. You should try to make your vocabulary 6_____ (big).
Read as many books as we can. There are a lot of books 7_____ (write) in easy English. You will enjoy them.
When you meet 8_____ new word, look it 9_____ in your dictionary. Your dictionary is your 10_____ (much)
useful book.
