健康成年人，一次失血超过（    ）就会危及生命．

我国地理学家胡焕庸提出了一条著名的人口地理界线是（　　）

Unlike a film, __________  a passive audience watches and hears __________  is happening on the screen, Real Cine puts you into the action and connects with your senses of sight, smell and touch in an active way.