I've come to the conclusion that I'm rubbish at saying “NO”.For example, last Sunday I was sitting at my desk alone，totally 1 in my own business, when a colleague (同事) of mine phoned and asked me to get involved in 2 with her. I found myself getting all caught up in her enthusiasm and didn't want to 3 her.So I said “Sure，why not?” and before the call had 4, I found myself agreeing without giving any 5 whether it was worth doing.

    We are both window-shoppers and 6 a whole day in the street. Actually, I hardly have any time to spare because I am very busy 7 my normal job—I've also got a book to write，a business to 8 and a family to look after. And then there are my other projects to 9— my voluntary work and a couple of other interesting investments(投资) that I want to 10. I'm going to have to be more ruthless(无情的)and not just agree to 11 people ask me to get involved in. In other words，I'm going to have to get 12 at saying “NO”.Usually I hate saying “NO”，because having to disappoint someone usually 13 me. Now I realize 14 I haven't learned to say “NO”. I've been feeling very 15.

    We had better do what we think is right and what we are 16 to do.Sometimes it does not seem that bad to 17 others.If I focus on the things I 18 want to be involved in，I'11 reach a much higher standard and do it quite 19. I'1l be enthusiastic and motivated(有积极性)and do something that is good for everyone and everyone will be happy.It'11 be a win-win 20. What's not good about that? So sometimes, we have to say “NO”.

若对任意$x\in A,y\in B,\left(A,B\subseteq R\right)$有唯一确定的$f\left(x,y\right)$与之对应，称$f\left(x,y\right)$为关于$x$、$y$的二元函数.现定义满足下列性质的二元函数$f\left(x,y\right)$为关于实数$x$、$y$的广义“距离”:
（1）非负性:$f\left(x,y\right)\ge 0$，当且仅当$x=y=0$时取等号；
（2）对称性:$f\left(x,y\right)=f\left(y,x\right)$；
（3）三角形不等式:$f\left(x,y\right)\le f\left(x,z\right)+f\left(z,y\right)$对任意的实数z均成立.
今给出四个二元函数:①$f\left(x,y\right)=x^2+y^2$；②$f\left(x,y\right)={\left(x-y\right)}^2$；③$f\left(x,y\right)=\sqrt{x-y}$；④$f\left(x,y\right)=\sin \left(x-y\right)$.

能够成为关于的$x$、$y$的广义“距离”的函数的所有序号是（     ）

阅读上海第五次（2000年）和第六次（2010年）全国人口普查数据材料。回答：

AB是⊙O的直径，点D在AB的延长线上，DC切⊙O于点C，若∠BAC=25°，则∠ADC等于（    ）