理想的基因转移方法应符合以下要求：A．外源基因能定向导入靶细胞 B．容易操作，易为临

       There are many kinds of tigers all over the world, and the 1____________
tiger is Northeast Tiger. Most Northeast Tigers live in the 2_____________ of
China, like Heilongjiang Province and Jilin Province. Because there are many
3_____________ and mountains, the tigers usually like living there. In the forests
 and mountains, there are fewer people 4____________ out and coming in. So the
tigers are very free. They can 5___________ after each other, catch and eat some