190个氨基酸组成的蛋白质由4条肽链组成，此蛋白质至少含有几个羧基和几个肽键？[

收益的稳定性是证券公司自营业务的特点之一．

（右足）を　（下ろして）ください。

其它条件不变，增大三通道喷煤管外风比例，火焰长度（）

A、增大

B、不变

C、减小

Today, roller skating is easy and fun．But a long time ago, it wasn’t easy at all. Before 1750, the idea of skating didn’t exist. That changed because of a man named Joseph Merlin. Merlin’s work was making musical instruments. In his spare time he liked to play the violin. Joseph Merlin was a man of ideas and dreams. People called him a dreamer.

One day Merlin received an invitation to attend a fancy dress ball（化装舞会）. He was very pleased and a little excited. As the day of the party came near, Merlin began to think how to make a grand entrance at the party. He had an idea. He thought he would get a lot of attention if he could skate into the room.

　　Merlin tried different ways to make himself roll. Finally, he decided to put two wheels under each shoe. These were the first roller skates. Merlin was very proud of his invention and dreamed of arriving at the party on wheels while playing the violin.

　　On the night of the party Merlin rolled into the room playing his violin. Everyone was astonished to see him. There was just one problem. Merlin had no way to stop his roller skates. He rolled on and on. Suddenly, he ran into a huge mirror that was hanging on the wall. Down fell the mirror, breaking to pieces. Nobody forgot Merlin’s grand entrance for a long time!

1．The text is mainly about ________.

A．a strange man              B．an unusual party

C．how roller skating began     D．how people enjoyed themselves in the 18th century

2．People thought Merlin was a dreamer because he _________.

　　A．often gave others surprises　　　　　 B．was a gifted musician

C．invented the roller skates　　　　　　D．was full of imagination

3．Merlin put wheels under his shoes in order to ________ .

A．impress the party guests　　　　　　B．arrive at the party sooner

C．test his invention　　　　　　　　　D．show his skill in walking on wheels

4．What is the main point the writer is trying to make in the last paragraph?

A．The roller skates needed further improvement. 

B．The party guests took Merlin for a fool.

C．Merlin succeeded beyond expectation.       

D．Merlin got himself into trouble.

溶液各处的密度、组成和性质不完全一样。