社会主义制度在我国基本建立起来的标志是（    ）

圆的周长和它的半径一定成正比例．

—I'm afraid not. I____ (visit) my grandparents in my hometown.

Dear Sir or Madam.

    One of the teachers at my school in New York tells me that your school has summer course. I am planning to go to Beijing next summer and would like to take a short one-month course.

    I am 18 years old and have been studying Chinese for six years. I was born in the USA and have never been to China before. I am going to study Chinese at the university. I think the course will help me improve my Chinese.

    Could you send me a catalogue(目录) and application form (申请表)? I would also like information about Beijing and other places nearby. I hope to do some travelling when the course is over.

    Thank you! I am looking forward to your reply

Very truly yours,

Jim Green

基因工程中，科学家常将细菌等微生物作为受体细胞，原因是（  ）