下肢静脉曲张的主要原因是（）
A.心脏功能不全
B.皮下脂肪减少
C.静脉瓣膜破坏
D.下肢肌肉收缩减退
E.胸腔负压作用减低
参考答案：C
计算机软件根据用途和性能，分为系统软件和（）。
A.操作系统
B.应用软件
C.数据库管理系统
D.语言处理程序
补全对话
A: Excuse me, sir, 1 hotel?
B: I am sorry I don't. You may ask that policeman over there.
A: Thank you 2 .
A: Excuse me. 3 ?
C: No, there isn't a hotel near here. But there is one near the Bank of China.
A: 4 .
C: It's about two kilometers away.
A: 5
C: You'd better take a taxi, because it's so late, there aren't any buses now.
A: 6 .
C: You're welcome.
etter take a taxi, because it's so late, there aren't any buses now.