The Following Data Represent the Means for Each Treatment Condition $\quad$

Multiple Choice

The following data represent the means for each treatment condition in a two-factor experiment. Note that one mean is not given. What value for the missing mean would result in no main effect for factor B? $\quad$$\quad$$\quad$$\quad$$\text {B1 }$$\quad$$\quad$$\quad$$\text {B2 }$
$\begin{array}{c}\begin{array}{lll}\text { A1 } \\\\\text {A2 } \end{array}\begin{array}{ | l | l| } \hline \\20\quad \quad&10\quad\quad \\\\40 & \\\hline\end{array}\end{array}$

A) 20
B) 30
C) 40
D) 50