A:so the school gave Mr. Smith leave
B:the school had given Mr. Smith leave
C:so Mr. Smith went on leave from work
D:Mr. Smith went on leave from work
如果实数x,y满足x-4y+3≤03x+5y-25≤0x≥1,目标函数z=kx+y的最大值为12,最小值为3,那么实数k的值为( )