--___a volunteer is great.A.Being B.To being

A being
成为一名志愿者是伟大的.Why can't choose B?if B was "to be" that will correct.Sorry,B is "to be". Can I Choose B?both are OK."to be "means you will become a volunteer "being" means you have already become a volunteer .for instance:"to see is to believe" or "seeing is believing"Thank you! Please look at the following.How many answers?--____a volunteer is great.--I think so. Some of us want ___volunteers for the Londen Olympics.A.Being ;beingB.To be ;beingC.Being;to beD.To be ;to beI prefer Cyou must choose the best onecan I Choose D? I'm a teacher. many student choose D.in my view,D is also correct.