The burden of proof always falls upon the arguer who has the benefit of presumption

Which of the following is described when two or more independent variables work in combination to produce an effect on a dependent variable?

a. a main effect b. an interaction effect c. a between-subjects effect d. a blocking variable

____________________ still has great value, if not more value than e-mail due to its personal nature and the effort required to create it and mail it.  

Fill in the blank(s) with the appropriate word(s).