Consider the Following Design and Assume No Interactive Testing Effect $\quad$

Consider the following design and assume no interactive testing effect is present:
(R) 0₁ X $\quad$ 0₂
(R) 0₃ $\quad$ 0₄
The difference represented by (0₂ - 0₁) - (0₄ - 0₃) estimates the EFFECT of

A) history.
B) maturation.
C) history, maturation and all other relevant sources jeopardizing internal validity.
D) experimental variable.
E) statistical regression.