A point (x, y) on the straight line Y=-X+6,

K=XY=X(-X+6)=-(X^2-6X+9-9)

=-(X-3)^2+9，

∵-(X-3)^2≤0,∴K≤9

The other end is easier. When crossing point C, K= 1×2=2.

∴K≥2。

Choose a.