第五大题：语法填空（共10小题；每小题1分，满分10分）
阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中词语的正确形式填空，并将答案填写在答题卷相应的位置上。
Opportunities don’t come often. They come every once in a while. Very often, they come quietly and go by without 66___________ (notice). Therefore, it is advisable that you should 67___________ (valuable) and treat them with care. When 68___________ opportunity comes, it brings a promise but never realizes it on 69___________ own.
