题目

Dear Santa Claus,My name is Amy. I am 9 years old. I have a problem at school. Can you help me, Santa? Kids laugh at me because of the way I walk and run and talk. I have cerebral palsy. I just want one day where no one laughs at me or makes fun of me.                                                                            love,                                                                                              某学生对函数f（x）=xsinx进行研究，得出如下四个结论：①函数f（x）在上单调递增；②存在常数M＞0，使|f（x）|≤M|x|对一切实数x均成立；③函数f（x）在（0，π）无最小值，但一定有最大值；④点（π，0）是函数y=f（x）图象的一个对称中心．其中正确的是（ ）A．③B．②③C．②④D．①②④