阅读下列材料，在空白处填入适当的内容(不多于3个单词)或括号内单词的正确形式。
    Friends are very important to teenagers．They not only give fun and excitement to each other butgive advice to one another．They can talk about lots of problems with the friends．As for parents，they are(sure)important but some teenagers often say their parents don't understand them．In large families，brothers and sisters often argueeach other for good things，such as delicious food or super toys．So most teenagers think friends are(important)people in their(life)．