vince knew that if he wanted to get into college, he'd have to keep his nose to the grindstone all senior year. what does "keep his nose to the grindstone" mean?

A,B,C,D?

A- Vince would have to apply for college.
B- Vince would have to work hard for a whole year.
C- Vince would have to wait patiently for an opportunity.
D- Vince would have to stay out of trouble for a whole year.​