If ` vec a ,\ vec b ,\ vec c` are three non-zero vectors, no two f thich are collinear and the vector ` vec a+ vec b` is collinear with ` vec c ,\ vec b+ vec c` is collinear with ` vec a ,\ t h e n\ vec a+ vec b+ vec c=`
a.` vec a` b. ` vec b` c. ` vec c` d. none of these