情景交际，选择适当的选项补全对话。

(D代表daughter,F代表father)

a.She's just a singer.,　　

b.Do you like her?,

c. Really?,

d. Who's she?,

e. I'm so excited.

D:Dad,I have good news to tell you.

F: What news?

D:A famous singer will hold a concert in Beijing on December 12th.

F:____　

D:She's a pop star from Singapore (新加坡).

F:____

D:Of course.____ She has decided this time to sing her most famous song It's Dark and Green Light.

F:I can't understand why you are so excited. ____

D:Oh,her songs are so wonderful and she's so lovely!

F:____ Your mum and I will buy you a ticket for her concert.

D: Thank you, Dad. You are too good.

①银氨溶液  ②溴的四氯化碳溶液  ③氯化铁溶液   ④氢氧化钠溶液．

在研究“一定电压下，电流与电阻的关系”时，电路如图所示．电源电压恒为3V，滑动变阻器上标有“15Ω 1A”字样．在a、b间先后接入不同阻值的定值电阻R，移动滑片P，使电压表示数为1.5V，读出电流表的示数．当20Ω的电阻接入a、b间时，电压表示数始终无法达到1.5V，其原因可能是（     ）