Let ` vec a , vec ba n d vec c` be three units vectors such that `2 vec a+4 vec b+5 vec c=0.` Then which of the following statement is true? a. ` vec a` is parallel to ` vec b` b. ` vec a` is perpendicular to ` vec b` c. ` vec a` is neither parallel nor perpendicular to ` vec b` d. none of these