A fundamental distinction between trend projection and linear regression is that:

A) trend projection uses least squares while linear regression does not.
B) only linear regression can have a negative slope.
C) in trend projection the independent variable is time; in linear regression the independent variable need not be time, but can be any variable with explanatory power.
D) trend projection can be a function of several variables, while linear regression can only be a function of one variable.
E) trend projection uses two smoothing constants, not just one.

C