第6部分：完形填空(第51～65题，每题1分，共15分)
下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。
Sharks Perform a Service for Earth’s Waters
51．A．meet B．attack C．recognize D．visit
52．A．research B．experiment C．analysis D．service
53．A．existence B．power C．mates D．skills
54．A．intelligenee B．shape C．activity D．size
55．A．fresh B．soft C．deep D．warm
56．A．whose B．which C．where D．that
57．A．for B．to C．as D．with
58．A．help B．food C．friends D．trainers
59．A．humor B．security C．touch D．smell
60．A．Droduced B．contained C．used D．added
61．A．those B．these C．other D．another
62．A．Drocesses B．systems C．ideas D．theories
63．A．escape B．recover C．return D．cut
64．A．hunting B．mental C．social D．swimming
65．A．weak B．little C．great D．few