In the opposite figure:
A fine lamina of a uniform thickness and density in the form of a right - angled triangle at \( B \) and of weight \( 30 \mathrm{~kg} . \mathrm{wt} \). acts at the point of intersection of its medians. Where \( A B=9 \mathrm{~cm} \).
\( B C=6 \mathrm{~cm} \). the lamina is suspended in a pin at a small hole near to the vertex B, a couple in its plane acted on it to make it in equilibrium when \( \overline{A B} \) is horizontal, then the algebraic measure of the moment of the couple \( =\ldots . . \mathrm{kg} . \mathrm{wt} . \mathrm{cm} \)
(a) 135
(b) -90
(c) -135
(d) 90
1.5 Degree