阅读下列材料，回答问题。 材料一随着东欧剧变、苏联解体，美苏对立的两极格局不复

阅读材料，解答问题．

当抛物线的表达式中含有字母系数时，随着系数中的字母取值的不同，抛物线的顶点坐标出将发生变化．

例如：由抛物线y=x²-2mx+m²+2m-1，…①

有y=（x-m）²+2m-1，…②

∴抛物线的顶点坐标为（m，2m-1）

即x=m …③

y=2m-1 …④

当m的值变化时，x、y的值也随之变化，因而y值也随x值的变化而变化

将③代入④，得y=2x-1…⑤

可见，不论m取任何实数，抛物线顶点的纵坐标y和横坐标x都满足关系式y=2x-1．

解答问题：

（1）在上述过程中，由①到②所用的数学方法是______，由③、④到⑤所用到的数学方法是______．

（2）根据阅读材料提供的方法，确定抛物线y=x²-2mx+2m²-3m+1顶点的纵坐标y与横坐标x之间的表达式．

汽车的牵引力是1000N且保持不变，先后通过200m的泥泞路段和200m的柏油路段，比较在这两个路段上汽车发动机做功的情况（　　）

A．在泥泞路段做的功多

B．在柏油路段做的功多

C．在两个路段做的功相同

D．无法判断

空气制动机的安全阀须分解检修，并进行试验，（）时排风，在160kPa前止排风，合格后铅封。

A、160kPa

B、170kPa

C、180kPa

D、190kPa

One of the most widely used routing protocols in IP networks is the Routing Information Protocol (RIP). RIP is the canonical example of a routing protocol built on the (71) algorithm. Routingprotocols in internetworks differ slightly from the idealized graph model. In an internetwork, the goal ofthe routers to forward (72) to various networks.
Routers running RIP send their advertisement abOLit cost every (73) seconds. A router also sends an up date message whenever an update from another router causes it to change its routing table.
Routing protocols use different techniques for assigning (74) to individual networks. Further, each routing protocol forms a metric aggregation in a different way. RIP takes the simplestapproach, with all link costs being equal (75) . Thus it always tries to find the minimum hop route. Valid distances are 1 through 15. This also limits RIP to running on fairly small networks.

A．1
B．15
C．16
D．length of the link

我国海洋捕捞业存在的突出问题是什么？