按要求写词语。     

1．写出三个描写夏天景物的词语：                、               、                。     

2．描写人物品质的词语：                   、                  、                    。     

3．照样子，写词语。     

     半信半疑：                                                                       

     兴致勃勃：                                                                     

     娓娓动听：                                                                    

简述弗农的产品周期理论的理论思路。