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（     ）

阅读短文，下列问题

    In the USA, there are many types of restaurants. Fast food restaurants are very famous. You can find McDonald's and Kentucky Fried Chicken everywhere.You look at a menu above the counter(柜台), and say what you'd like to eat. You pay the person who serves you. There's no need to leave a tip. In a coffee shop you sit at the counter or at a table. You don't need to wait for the waitress to show you where to sit. She usually brings you coffee when you sit down. You tell her what you'd like to eat and she brings it to you. You pay the cashier (出纳员) as you leave.A diner is like a coffee shop but usually looks like a railway carriage (火车车厢). In a family restaurant, the atmosphere is casual, but the waitress shows you where to sit. Often the waitress tells you her name, but you don't need to tell her yours. You add an extra fifteen percent to the bill as a tip. In top class restaurants, you need a reservation (预定) and you need to arrive on time. The waiter shows you where to sit. If you have wine, he may ask you to taste it. You can only refuse it if it tastes bad. When you get your bill,check it and then add fifteen to twenty percent to it as a tip for the waiter.

若b<0，则二次函数y=x²-bx-1的图象的顶点在（    ）