下面问题的对偶问题是（ ）原问题：max Z=50x1+100x2约束： x1+x2≤300 2x1+x2≤400 x2≤250x1, x2 ≥0

A、max f=300y1+400y2+250y3y1+2y2≥50y1+y2+y3 ≥100y1, y2, y3 ≥0
B、min f=300y1+400y2+250y3y1+2y2≤50y1+y2+y3 ≤100y1, y2, y3 ≤0
C、min f=300y1+400y2+250y3y1+2y2≥50y1+y2+y3 ≥100y1, y2, y3 ≥0
D、min f=50y1+100y2y1+2y2≥300y1+y2+y3 ≥400y1, y2, y3 ≥0