If ` vec a` and `vec b` are orthogonal unit vectors, then for a vector ` vec r` noncoplanar with ` vec a` and `vec b` , `vec rxx veca` is equal to
a. `[ vec r vec a vec b] vec b-( vec r. vec b)( vec bxx vec a)`
b. `[ vec r vec a vec b]( vec a+ vec b)`
c. `[ vec r vec a vec b] vec a-( vec r. vec a) vec axx vec b`
d. none of these