公钥体制用于大规模电子商务安全的基本要素是（）

保证商业服务不可否认的手段主要是（）

名词解释：第三方物流

完型填空(本大题共 10 小题，每小题 1 分，共 10 分)从选项中选择一个最适合短文的选项，错选、多选或未选均无分。 

Jogging is becoming one of the ___21___ growing activities in the nation. More people are starting jog each year. This, however, is a recent occurrence. It was not long ago __22____ joggers were rare sight. Some people thought that they looked quite strange. As __23____ people began jogging the sight of them became more common. There are many reasons ___24___ a person to take up jogging. First, it is a good form of exercise. This is __25____ it involves the use of many parts of the body. The exercising of the heart is of great benefit. This is one of the most important parts of the human body. If the heart does not get the needed exercise, serious problems can occur. ___26___ reason for jogging is for enjoyment. ___27___ being good for people, it can also be very much fun. Part of the fun of jogging comes from the opportunity it provides for meeting other people who enjoy it. One ___28___ that should be considered ___29___ a person takes up jogging as a form of exercise of enjoyment is that it is not for everyone. People should first check 30 their doctors and be prepared to find another form of exercise if jogging is not recommended.

21. A. fastest B. quickest C. most D. best 

22. A. when B. while C. that D. as 

23. A. many B. those C. more D. the 

24. A. of B. with C. for D. in 

25. A. that B. why C. reasonable D. because 

26. A. A B. Other C. The other D. Another 

27. A. Except B. Except for C. Besides D. Regardless of 

28. A. matter B. event C. thing D. occurrences 

29. A. of B. that C. what D. when 

30. A. out B. over C. on D. with

__________ were naturally a musical family .