污染物
参考答案：在特定的环境中达到一定的数量或浓度并持续一定实践，有易变性，发生一定的转化，进入环境后能够直接或间接危害人类的物质。
枕骨大孔疝的主要临床表现有（）
A.剧烈头痛，频繁呕吐
B.颈项强直
C.血压增高，脉搏缓慢
D.呼吸骤停
E.以上均对
IP multicasting is a set of technologies that enable efficient delivery of data to manylocations on a network．Rather than making multiple copies of a message intended to bedistributed to multiple recipients at the (71) of origin of a message，multicasting initiallysends just one (72) and does not copy it to the individual recipients until it reaches theclosest common point on the network，thereby (73) the bandwidth consumed．Networkperformance is significantly (74) because it isn’t bogged down with the processing andtransmission of several large data files；each receiving computer doesn’t have to (75) thetransmitting server for the file．
A．loop B．find C．search D．query
