our children ____live to see the next century ,what change will they see?为什么填should不填could
假设一种将来未曾发生的事,此处用虚拟语气should而不用could.
As is natural,a man as old as he ___be very forgetful为什么填can
表示一种自然的规律,或者已成为定理,公理了,用一般形式can,是一种固定用法.