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
D
ComradeLi________beinBeijingbecauseIsawhimintownonlyafewminutesago.
Howlong________they________(dig)theditch?
Thisdiploma(毕业文凭)________thatyouhavecompletedhighschool.
设，则数列sn有界是数列an收敛的（）
EachyearintheUnitedStates,manyblackteenagers_______ofschool,eitherbecausetheycannotkeepuportheyhavetoworktosupporttheirfamily.
