若实数对（x，y）满足x2+y2=4，则xy的最大值为______．
∵x2+y2=4≥2xy
∴xy≤2
当且仅当x=y时取等号
故答案为：2
A few days after the interview, I received a letter _____ me admission to the university. [ ]
A. offering
B. offered
C. having offered
D. to be offered
Some people strongly object _____ private cars.[ ]
A. to develop
B. to developing
C. developing
D. development
