Regarding the Practice of Manipulating Two Variables Simultaneously One Should

Multiple Choice

Regarding the practice of manipulating two variables simultaneously one should conclude:

A) This is acceptable practice as researchers now believe that we can tease out interactive effects.
B) This is acceptable practice because we can statistically manipulate individual variables.
C) This is unsound practice because in so doing we are unable to assess effects of either variable.
D) This is unsound practice because in so doing we confuse the participant as a threat to validity.