As used in this passage，the word “valid” in the second paragraph means______

历史上许多科学家与发明家曾充分利用白日梦。

But your own books belong to you；you treat them with that affectionate intimacy that annihilates(歼灭；消灭)formality. Books are for use，not for show；you should own no book that you are afraid to mark up，or afraid to place on the table，wide open and face down. A good reason for marking favorite passages in books is that this practice enables you to remember more easily the significant sayings，to refer to them quickly，and then in later years，it is like visiting a forest where you once blazed(在树上做标记)a trail. You have the pleasure of going over the old ground，and recalling both the intellectual scenery and your own earlier self.

一个社会的人口增长率越接近零，它的人口就可能越老龄。

只是在近几年天文学家才开始具体研究黑洞。

她说在伦敦她干活的家里，她觉得就像个奴隶。

人的一生如果没有想象和梦幻，他的一生将是十分贫乏的。

It would be completely irresponsible and unethical to use drugs on people that______(not test)thoroughly on animals.

We know that we are getting even older，and that the______(near)a society approximates to zero population growth，the older its population is likely to be.

Although managers cannot predict the future，many of their decisions require that they______(consider)possible future events.

______(affect)with a serious disease，Van Wendel was no longer able to speak clearly and he knew there was no hope of recovery.