36. If `vec a, vec b,vec c and vec d` are unit vectors such that `(vec a xx vec b)`.`vec c xx vec d= 1` and `vec a.vec c =1/2` then a) `vec a, vec b and vec c` are non-coplanar b) `vec b, vec c ,vec d` are non -coplanar c)`vec b, vecd` are non parallel d) `vec a , vec d` are parallel and `vec b, vec c ` are parallel