进入初中后，有很多新鲜的事物，让我们好奇，令我们欣喜。班主任告诉我们：走进中学，也就是走进了青春期，你会发现自己每天都在变。还提醒我们：自己的事情自己独立完成，要学会自立；和同学友好相处，朋友越多越好哟……

设平面$\alpha$与平面$\beta$相交于直线$l$，直线$a$在平面$\alpha$内，直线$b$在平面$\beta$内，且$b\perp l$，则“$b\perp a$”是“$\alpha \perp \beta$”的（  ）

A

    It's a beautiful day in summer. There are many people on the beach. Mrs Guo and her two sons are on the beach, too. Look! Mrs Guo is swimming in the sea. She can swim fast. What are her sons doing? They are playing volleyball on the beach. There's a man under the umbrella. He's reading a book. Look! A little girl is collecting shells on the beach. She's very happy.

氨水是一种的常用的化学试剂．