当关系只和S自然联接时，能够把R和S原该舍弃的元组放到结果关系中的操作是(17)。

(50)

(49)

(48)

For each blank, choose the best answer from the four choices and write down on the answer sheet.

 (71) is a professional organization of individuals in multiple professions which focuses on effort on lower-layer protocols.

 (72) is the popular backbone technology for transmitting information at high speed with a high level of fault tolerance which is developed under the direction of ANSI.

 (73)functions with two layers of protocols. It can connect networks of different, speeds and can be adapted to an environment as it expands.

(47)

(75)is　a　protocol　that　a　host　uses　to　inform.　a　router　when　it　joins　or　leaves　an　Internet　multicast　group。

(74)is　a　network　layer　device　that　supports　multiple　LAN　interfaces　and　segments　LANs　into　smaller　collision　and　broadcast　domains。

(73)is　an　identifier　of　a　web　page。

(71)is　a　device　which　enables　digital　microcomputers　to　communicate　across　analog　telephone　lines。

(72)is　an　error　detection　code　that　most　data　communication　networks　use。