下列关于氨基酸和蛋白质的叙述，错误的是[ ]

A．甲硫氨酸的R基是-CH₂-CH₂-S-CH₃，则它的分子式是C₅H₁₁O₂NS

B．酪氨酸几乎不溶于水，而精氨酸易溶于水，这种差异是由R基的不同引起的

C．n个氨基酸共有m个氨基，这些氨基酸缩合成的一条多肽中的氨基数必为m-n

D．甜味肽的分子式为C₁₃H₁₆O₅N₂，则甜味肽一定是一种二肽

If you have decided to study abroad, you might be looking for effective ways to prepare your language skills before you depart. Some students attend ESL (English as a Second Language) classes at local schools while others practice with   1  conversational partner. However, for those in situations   2  these two options are not available, studying ESL online might be the next alternative.

During the past decade, many websites have sprung up on the Internet,   3  (provide) ESL activities to help people practice the major language skills  4  grammar, reading, and vocabulary. Perhaps the  5  (great) interests for students are websites that focus on listening and speaking skills. In the past, listening to audio online was a very slow process, but since high-speed connections appeared, things  6  (change) greatly. A simple Internet search using the term “ESL” will bring up thousands of relevant websites out there, but not all the sites are the same. 7  you want to find a site that is suitable for you, you should look among those that combine good contents with a user-friendly interface and style.

Finally, no matter which site you end up visiting, you should set some goals  8  your study.   9  might include the number of vocabulary you want to learn, specific grammar structures you want to review,  10  even how many minutes you want to read aloud from a newspaper in English.