Would it surprise you to know that poisonous snakes can be milked? Taking the poison from a snake's sharp teeth is called milking it. Milking snakes is the sole job of some snake scientists. It is their only job, and it is a dangerous one.

What does "sole" mean in this passage?

A.just one.

B.a problem to solve.

C.very important.

D.a kind of animal.​