Let the sliding distance of the small iron block be x, the gravity of the iron block be mg, and the length of the light plate be l, and the balance can be obtained according to the moment with O as the fulcrum: mgx=TLsinθ.

D: t = mgx？ Lsinθ

Let the magnitude of the force exerted by the hinge on the plate be F, and decompose F into horizontal right component Fx and vertical upward component Fy, which are obtained from the equilibrium conditions: Fx=Tcosθ,

Tsinθ+Fy=mg

Simultaneous solution: Fx=mgcotθ? xL，Fy=mg( 1-xL)

Let xL=y

F=F2X+F2y=mgcotθ2y2+( 1？ y)2=mg(cotθ2+ 1)y2？ 2y+ 1

According to mathematical knowledge, f first decreases and then increases.

So choose d