$A$ force $F_1 = A \hat{j}$ is applied to a point whose radius vector is $r_1 = a \hat{i}$, while a force $F_2 = B \hat{i}$ is applied to the point whose radius vector is $r_2 = b \hat{j}$. Both the radius vectors are determined relative to the origin of the coordinate axes $O$. The moment of the force relative to $O$ is

• A
  $(a A - b B) \hat{k}$
• B
  $(a A - b B) \hat{j}$
• C
  $(a b - A B) \hat{k}$
• D
  $(a B - b A) \hat{j}$