$\Lambda$ wooden plank of length $\mathrm{lm}$ and uniform cross-section is hinged at one end to the bottom of the tank. 'Ihe tank is filled with water upto a height of $0.5 \mathrm{~m}$. 'lhe specific gravity of the plank is $0.5$. The plank makes an angle of $45^{\circ}$ with the vertical in the cquilibrium position. $A$ is the midpoint of the plank. $B$ is the middle point of the dipped parl of the plank. Which of the following are correct?
(a) The weight of the plank acts downwards at $\Lambda$
(b) The buoyant force acts through $B$
(c) $O B-\frac{1}{\sqrt{2}} m$
(d) Both (b) and (c)