（）爱岗敬业的最高要求：投身于社会主义事业，用有限的生命投入到无限的为人民服务中去。
参考答案：对
—Flight MU257 _______________ . I must be off now.
—Have a nice trip.
A．is being announced
B．has announced
C．was announcing
D．had been announced
设函数f(u，v)具有二阶连续偏导数，z=f(x，xy)，则
______.
______.