—What about ____ book?　    —It’s too difficult ____ book for beginners.　   

(2013·高考陕西卷)Marco Polo is said to have sailed on________Pacific Ocean on his way to Java in________thirteenth century.

We can never expect         bluer sky unless we create         less polluted world.

(2013·无锡一模)The education of ________ young is always ________ hot and serious topic.

I don’t think the experiment is ______ failure, because it has provided us with _____ valuable experience for our future tests.