河姆渡原始居民和半坡原始居民生活的共同点是（   ）

补全对话。

Ann: What are you going to do this afternoon?

Ken: I'm going to buy some DVDs about Quanzhou Maritime Silk Road.

Ann:____ Why not go to the Film Fans' Room?

Ken: The Film Fans' Room? ____

Ann: It is a place where students can exchange their DVDs with others. I think you can get what you want there.

Ken: ____How can I get there?

Ann: It's next to Donghu Cinema. You can take the No. 13 bus.

Ken: OK. ____

Ann: There is one just outside Huaqiao University. You can get on the bus there.

Ken: Thank you very much.

Ann: ____

我国交流电的周期是____s，频率是50Hz，在一个周期里电流的方向变化____次．

半径不等的两定圆$O_1$、$O_2$无公共点（$O_1$、$O_2$是两个不同的点），动圆$O$与圆$O_1$、$O_2$都内切，则圆心$O$轨迹是（   ）