如图是巴斯德著名的“鹅颈瓶”实验示意图．甲、乙两瓶内装有等量的同种肉汤，甲瓶煮沸，乙瓶不做处理．一段时间后，其中一瓶仍然保鲜，另一瓶变质；接着，将保鲜那瓶的瓶颈打断（如丙图），数日后，瓶中的肉汤也变质．下列说法中错误的是（　　）

假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。每处错误仅涉及二个单词的增加、删除或修改。

增加：在缺词处加一个漏字符号(∧)，并在其下面写出该加的词。

删除：把多余的词用斜线（\）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1.每处错误及其修改均仅限一词；

    Your problem is common one among middle school students. Maybe the following advices can help you. First in all, believe in yourself. Your greatest problem is that you lack(缺少) self-confidence. The first thing you must do is to smile at your classmate. One smile speaks loud about your wish to make friends than any word. Your smile will show that you are friendly to him. Next, trying talking with a student who is as shy as you or who share the same interest as you. You can discuss your studies with a classmate, and you can also talk about your hobbies as well. Unless anyone is in trouble, you should be ready to help him or her. Once you have confidence, you can make as many friends as possibly.

已知$a=\sin {15}^{°}\cos {15}^{°}$，$b={\cos }^2\frac{\mathrm{\pi }}{6}-{\sin }^2\frac{\mathrm{\pi }}{6}$，$c=\frac{\tan {30}^{°}}{1-{\tan }^2{30}^{°}}$，则$a,b,c$的大小关系是（  ）