Needle Is a Private Laboratory That Performs More Than 100

Needle is a private laboratory that performs more than 100 different tests and analyses. Four tests require the use of a radiological counting machine that can supply 6,000 hours per year. Information on the four lab tests are as follows:
$\begin{array}{lrrrr} & \text { Test A } & \text { Test B } & \text { Test C } & \text { Test D } \\\text { Charging rate } & \$ 50 & \$ 20 & \$ 80 & \$ 70 \\\text { Variable cost } & 10 & 10 & 60 & 30 \\\text { Machine hours } & 2 & 1 & 0.5 & 0.25\end{array}$
-Refer to the Figure.What is the contribution margin per hour of machine time for Test B?

A) $10
B) $20
C) $40
D) $100