“又灌溉三郡，开稻田，于是蜀沃野千里，号为陆海，旱则引水浸润，雨则杜塞水门。”这是对哪一水利工程的评价（）

A.都江堰

B.郑国渠

C.坎儿井

D.白渠

Originally introduced by Netscape Communications, (48) are a general mechanism which HTTP Server side applications, such as CGI (49) , can use to both store and retrieve information on the HTTP (50) side of the connection. Basically, Cookies can be used to compensate for the (51) nature of HTTE The addition of a simple, persistent, client-side state significantly extends the capabilities of WWW-based (52) .

51（）

A.fixed 

B.flexible 

C.stable 

D.stateless