Costs for regulation of business actually are a hidden tax severely reducing the competitiveness of domestic businesses ______ when they face an increasingly global marketplace.
A.
at times
B.
at present
C.
at once
D.
at a time
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【简答题】已知圆O:x 2 +y 2 =r 1 2 (r 1 >0)与圆C:(x-a) 2 +(y-b) 2 =r 2 2 (r 2 >0)内切,且两圆的圆心关于直线l:x-y+ 2 =0对称.直线l与圆O相交于A、B两点,点M在圆O上,且满足 OM = OA + OB (1)求圆O的半径r 1 及圆C的圆心坐标; (2)求直线l被圆C截得的弦长.