设等比数列$\left\{a_n\right\}$的公比$q=2$，前$n$项和为$S_n$，则$\frac{S_4}{a_2}$的值为（   ）

The boy is       to his teacher.

氟化硅（SiF₄）是有毒的无色气体，可用于水泥和人造大理石的硬化剂等，制取氟化硅（SiF₄）的化学方程式：2CaF₂+H₂SO₄+SiO₂CaSO₄+SiF₄↑+H₂O，完成下列填空：

    The verb“ chat” means to talk (to someone)in a friendly, informal(非正式的)way. Today there are many ____ on the Internet where you can chat. You can chat with people all over the world, in many languages and about many subjects or topics. These places are ____ called “chatroom”. Sometimes you need to register( 注册)to enter a chatroom. This means that you need to provide a username(用户名)and possibly your e-mail ____.Your username doesn't have to be your ____ name. It can be any name you want. When you have your username, other people in the chatroom will call you by that name and ____ else can use it. Chatting is a good way to ____ your informal English. Messages in a chatroom are usually short sentences. Sometimes ____ are not even sentences, ____ just a few words that are not really grammatically(语法的)correct. It's a typical way in which we ____ in a quick conversation with friends. To save time, people often use abbreviations like IDK(I don't know).So don't go to a chatroom to practice “perfect” English. Go to a chatroom to practice informal English and find ____. You can even use a chatroom to help improve your typing skills if you want.