题目

Very often we will have to speak to a large audience. For example, we may need to give a talk, in which case most of us will be shy and embarrassed. There is no   41  for this. If a few simple rules are  42 , anyone can speak well in  43 . First of all, the talk should be well  44 . In this way we  45  a good command of what we are going to say and  46  up our confidence. By working hard we can get some necessary facts or information  47 ; as a result, we feel much  48 .    There are  49  things we should know when we are preparing our 50 . The first one is what we are当桑树等植物的茎被折断时，从茎的断面上会渗出白色的汁液，流出该汁液的结构是（?? ）A.导管??? B.筛管???? C.形成层???? D.髓