C is correct. The work-force relation is given by W=F·d, where F is force vector, and d is the displacement vector. The dot is the dot product, which is a measure of how parallel the two vectors are. It can be restated as the product of two vector magnitudes times the cosine of the angle between them. Therefore work is a scalar, not a vector, since the dot product returns a scalar.
The magnitude of the gravitational force depends inversely on the square of the radial distance between the centers of the two masses. Thus, essentially, the force can only fall to zero, when the denominator that is r becomes infinite.
