In Literature Reviewing, the Issue of "Mixing Apples and Oranges

In literature reviewing, the issue of "mixing apples and oranges" refers to

A) making inappropriate comparisons across different types of moderator variables.
B) not including a mediating variable when doing so would provide clearer results.
C) inappropriately combining results of studies that use different independent variables, dependent variables, or other characteristics of the studies being reviewed.
D) including studies that did and did not have significant results.