In the study of physics, what distinguishes a scalar from a vector?
a) Scalars have both a magnitude and a direction, but vectors have only a magnitude.
b) Scalars are specified with a single number, but vectors are specified using both a magnitude and a direction.
c) Nothing—the terms "vector" and "scalar" are different names for the same thing.
d) A scalar is a dimensionless number, while vectors are numbers that have dimensions.