It is Sunday tomorrow. We are going to watch a football game. It is between a Japanese team and a Chinese team. The football game will begin at 4：00 in the afternoon. Our teacher, Mr. Wang will watch the football game with us. We will go there by bus. We think we can come back at about 6：00 in the afternoon.

一只雌猫一次产了三只小猫，这三只小猫在毛色上不完全相同，则说明物体具有（  ）

据统计，清末版本目录《增订四库简明目录》共著录历代书籍七千七百四十八种，约计不同版本两万部，其中活字印本只有二百二十部，约占总数的百分之一。上述现象出现的主要困素是（    ）