选答语。
①What's your name? ____ A. I'm from China.
②Where are you from? ____ B. He's my grandpa.
③Who's that man?____ C. My name is Tutu.
下列四类函数中,具有性质“对任意的x>0,y>0,函数f(x)满足fxy=fxy”的是( )