80名同学参加体质测查，全部合格，且有50人达到优秀．这批同学体质测查的合格率是

     We all know that some things are obviously right. For example, it is right to be   1   to other people. It is also right to look after the environment. Some things are   2   wrong, too. For instance, we should not hurt or bully (欺负) others, nor
should we litter. Rules often tell us what is right or wrong.
    Rules can help the public make the right   3  , and remain safe. Car divers have to obey traffic regulations that tell them
the right things to do on the road to avoid crashes. Cyclists who give signals before turning or stopping help prevent   4  .
    If people follow rules without taking other matters into consideration, it will be   5   for them to form what is sometimes
called a "black and white" view. For example, they may believe that people should always tell the truth, and that lying is   6   acceptable. Such people always stick to their views, even if it means that they may get into   7  .
    Sometimes it may not be so easy to know   8   what is right or wrong. Some people choose not to eat meat because they believe that it is   9   to eat animals, but other argue that they can eat meat and   10   be kind to animals; some insist that
stealing is always wrong, but others think that one does not need to feel so   11   when stealing some food to eat, if he lives in a really poor area and he is   12  .
    Rules help us live together in harmony, because they show us the right way to   13   others. However, some people
argue that rules may be   14  , having observed that rules change all the time, and that some schools have some
regulations and others have different ones -- so who is to   15   what is right ?