设函数 f (x, y)为可微函数，且对任意的 x, y 都有
则使不等式
成立的一个充分条件是
A、
B、
C、
D、
B、
C、
D、
正确答案：
D
答案解析：
Each year in the United States, many black teenagers _______ of school, either because they cannot keep up or they have to work to support their family.
The address book ________ the email addresses of your friends or groups of colleagues.