单选题 　Passage 1 　　Each student at an engineering college in Canada gets a steal ring upon graduation as well as graduation certificate. 　　There is a story behind the unusual ring. Several years ago, a graduate from this engineering college designed an iron-steel bridge. Unfortunately, because of mistakes in his design, the bridge collapsed soon after it came into use. 　　This event shook the engineering college. And the head of the college decided to take back the framework of the iron-steel bridge and use it as the material for making rings to give the students upon their graduation. The college wants the students to remember this lesson forever so as to avoid making such mistakes again. Upon graduation each student is given a steel ring as a