A：21
B：22
C：23
D：24
基因工程的正确操作步骤是（ ）
①使目的基因与运载体结合
②将目的基因导入受体细胞
③目的基因的表达和检测
④提取目的基因．
选出相对应的答句。
①Touch your mouth.____ A. Four books.
②How many books?____ B. This is my mouth.
③Here you are. ____ C. Here you are.
④Give me a pencil, please.____ D. One, two. Two pencils.
⑤How many pencils?____ E. Thank you.
地球绕着地轴不停地旋转叫____ ，绕着太阳不停地旋转叫____ 。
Students don't go to school ________ Sundays.
