What Is the Difference Between a Blocking Variable and a Covariate

What is the difference between a blocking variable and a covariate?

A) Blocking variables result in main effects whereas covariates do not.
B) Blocking variables are manipulated variables, but covariates are simply measured.
C) Blocking variables are categorical variables and covariates are continuous variables.
D) Blocking variables are used to statistically control for variance due to that variable, whereas covariates represent multiple independent variables and are the focus of the main effects in an experiment.