Dolphins become more and more 41 animals at zoo.They are mor

Dolphins become more and more 41 animals at zoo.They are more willing to work 42 their trainers than other animals are,and can do tricks,which pleases children.But they don't like doing the same trick twice.This is one reason 43 believing that they are clever.Dolphins live in a 44 -organized society.They look after each other when they are ill,and keep the weakest 45 in the big family,46 we do.Some scientists know that dolphins have a 47 ,but it seems that they pass on information by means of sounds without 48 "words." They have a kind of radar ( 雷达 ) just like "bats." Their hearing is much more sensitive (敏感的) than 49 ,and the most important task of their brains is to send 50 signals and receive them.4l.A.popularer B.population C.popularized D.popular 42.A.at B.with C.as well as D.better with 43.A.of B.for C.to D.why 44.A.best B.good C.well D.more better 45.A.safe B.safety C.safely D.unsafe 46.A.and B.as C.however D.like 47.A.language B.eyesight C.hearing D.sound 48.A.need B.needing C.use D.use to 49.A.our B.us C.ours D.Theirs 50.A.out B.off C.down D.by