阅读理解

    What's your hobby? Reading，playing sports or going to the movies?Hassan has an interesting hobby．He likes collecting tropicalfish(收集热带鱼)! He keeps this hobby for 7 years．Now he is 14．There is a big aquarium(养鱼缸)in his room．It is from his parents for his 13th birthday．There are many fish in it．All of the fish are cute and beautiful．

    Hassan often travels around the world(周游世界)with his family．He loves it because he can see the underwater world in many places．He always loves the colorful underwater world．Under the water,he can see lots of tropical fish．Sometimes he can see sharks(鲨鱼)．He wants to keep one or two small sharks in an aquarium,but he can't do it at home．So Hassan goes to help in the zoo on the weekend．There he can see a lot of sharks and some other big fish．Hassan likes working in the zoo，because he can work with his favorite animals and leam more about them．

根据材料内容选择最佳答案。

孩子们____的六一儿童节终于到了，校园里处处____，处处____，同学们个个都显得____。

2015年5月拉萨市某酒店入住人数是1500人，随着旅游旺季的到来，该酒店7月预计入住人数为2160人，求该酒店6月、7月预计入住人数的月平均增长率．设预计月平均增长率为x，则根据题意可列方程为（　　）

杂交育种的基本原理是