若4x²+mxy+25y²是完全平方式，则m=____．

明明今年a岁，东东今年a+4岁，再过x年，他们相差（　　）岁．　

如图甲，电源电压恒为9V，滑动变阻器的最大阻值为100Ω，电流在0.1A~0.4A之间时电子元件均能正常工作．若通过此电子元件的电流与其两端电压的关系如图乙所示，下列判断正确是                                 (    )

    I'm Helen . I'm an English girl . I'm a student . I'm in Class 1, Grade 1. I get up at six o'clock . I go to school at seven o'clock. We have three classes in the morning and two in the afternoon . We go home at four thirty . My father is a policeman . My mother is a driver . I love my father and mother .