The Great Wall of China is more than 6,000 kilometres long. It winds ____ (it) way from west to east, across deserts, over mountains, through valleys____at last it reaches the sea. The Great Wall has a history of over twenty centuries. The first part of it ____(build) during the Spring and Autumn period. During the Qin Dynasty, ____(keep) the enemy out of his empire, Emperor Qin Shi Huang had all the walls ____(join) up. Thus, the Great Wall came into being. The Great Wall is wide enough at ____top for five horses or ten men to walk side by side. Along the wall are watchtowers, ____soldiers used to keep watch. Fires were lit on the towers as a ____ (warn) when the enemy came.

    It was ____(extreme)difficult to build such a wall in the ancient days without any modern machines. All the work was done ____hand. Thousands of men died and were buried under the wall they built. The Great Wall was made not only of stone and earth, but of the flesh and blood of millions of men.

一个不透明的口袋中有三个完全相同的小球，把它们分别标号为1，2，3，随机摸出一个小球，然后放回，再随机摸出一个小球，两次摸出的小球标号的和为5的概率是（　　）