Mr. Smith is an English teacher. He comes from America. He has a daughter. Her name is Amy, and she is a good engineer in New York. Mr. Smith likes traveling very much, and he can speak Spanish very well. Now, he is in China, so he wants to learn some Chinese. He works very hard at his Chinese, and goes to Chinese classes every evening. He has a lot of friends in China and they often help him. He likes to talk with them. He tells his daughter that he can speak good Chinese, and he doesn’t have any trouble（麻烦） with his Chinese, but the Chinese have a lot.

如图，在平行四边形ABCD中，对角线AC,BD相交于点 O，若BD,AC的和为18 cm， $\text{CD}:\text{DA}=2:3$ ，△AOB的周长为13 cm，那么BC的长是（    ）

发电厂发电机的输出电压为U₁，发电厂至用户的输电导线的总电阻为R，通过输电导线的电流为I，用户两端得到的电压为U₂，输电导线上损耗的功率可表示为（   ）

下列选项中有错的一项是(    )