方程组 （1）$\left\{\begin{array}{c}x-2=0\\ x+3y=6\end{array}\right.$（2）$\left\{\begin{array}{c}x+y=4\\ x-z=1\end{array}\right.$ （3）$\{\begin{array}{c}x-y=2\\ x+y=4\\ 2x-3y=1\end{array}$ （4）$\left\{\begin{array}{c}x-3=y\\ xy+1=4\end{array}\right.$  中，属于二元一次方程组有（　　）

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PRIVATE ENGLISH LESSONS

    An experienced teacher is offering private lessons in English．

Please email me at the address below and tell me．

    ·Your age

    ·Reasons for learning English

    ·Your level of English

    ·Areas that you need to focus on--grammar，listening，speaking，reading，writing，etc．

     Mrs Daniels(daniels@mail．corn)

Dear Mrs Daniels，

         I recently saw your advertisement on the university website offering private English lessons．I'm very interested in your private lessons and would like some more information．I am a student at Aston College here in Hong Kong studying English，so my language level is already quite good．

     However，I feel that I need to improve my listening and speaking skills．This is because I'm planning to study at London University in England next year and I think your lessons will help me to live and study abroad．

     Could you send me some more information about the lessons offered，such as where and when they will be held and how much they cost? Also，I would like to know whether you teach students in groups or one-on-one．

     I would be glad to supply any further information you might need．

                                                                                                                                 Regards，

                                                                                                                                                                 Li Hua

如图，圆内接四边形ABCD是由四个全等的等腰梯形组成，AD是⊙O的直径，则∠BEC的度数为（    　）

若x²+cx+6=（x+a）（x+b），其中a，b，c为整数，则c的取值有（  ）

由于乙型肝炎病毒因专性寄生及胞内寄生等原因不能用动物细胞培养，因此无法用细胞培养的方法制造疫苗。自1979年对乙型肝炎病毒基因组的DNA测序完成后，得知病毒的核心蛋白和表面抗原蛋白的氨基酸序列。下图是应用生物工程生产乙肝疫苗的示意图，据图分析下列叙述正确的是                                 （   ）

发醇 ③

引入 ②

分离 ①

      乙肝病毒有关基因细菌大量生产疫苗