下面关于地球形状的最佳表达是（　　）

Look ________ the cat

    Now you are in a dinosaur ____park. Coming here, you will find a kind of old ____, dinosaur, which died out millions of years ago. In our park, we are happy to show you ____ dinosaurs, which lived in the Jurassic. The four-storied-building-high grass-eating dinosaurs are submissive (温顺的) and kind. You can ____ towards them and touch them. But the ____  of them are dangerous. The four-legged meat-eating dinosaurs are very cruel. Please don't ____ them and don't try to give them food. The ____ here is very thick and full of all kinds of animals that lived in the Jurassic. Without any special____and the leading of the guide, please don't step into it. OK, I'm sure that our park will ____anyone who comes here and you will ____a lot from it.

如图所示，点P，Q，C都在直线AB上，且P是AC的中点，Q是BC的中点，若AC=m，BC=n，则线段PQ的长为（　　）