下列不属于生态系统的是  （  ）

提示词：listen to, mother's love, care about, communicate with, get along with, smile

要求：60~80词，可适当发挥。

Dear Wang Ning,

    I'm afraid I've got a big problem recently. My mother talks too much to me. She always tells me,“Be careful while crossing the street.”“Put on more clothes.”“Did you do a good job at school?”and so on. I'm annoyed（烦恼的）.What shall I do?

Yours,

Tina

甲灯额定电压为6V，乙灯额定功率为2W，甲、乙两灯的电阻分别为R_甲和R_乙，且R_甲>R_乙，并保持不变，将它们以某种方式连接到电压为U_A的电源上时,两灯均正常发光,将它们以另一种方式连接到电压为U_B的电源上时，乙灯正常发光,甲灯的实际功率为额定功率的4/9，则电源U_A两端的电压为____ V。