If ` hat a , hat b ,a n d hat c` are three unit vectors, such that ` hat a+ hat b+ hat c` is also a unit vector and `theta_1,theta_2a n dtheta_3` are angles between the vectors ` hat a , hat b ; hat b , hat ca n d hat c , hat a` respectively, then among `theta_1,theta_2,a n dtheta_3dot` a. all are acute angles b. all are right angles c. at least one is obtuse angle d. none of these