How does the iron concentration of cancerous and non-cancerous tissues compare? The file FeConTissues contains data from a study aimed at assessing whether trace metal concentrations, which influence metabolism as both essential and non-essential elements, are raised or lowered in cancerous tissues.

a. At the 0.05 level of significance, is there evidence of a difference in mean iron concentration between cancerous and non-cancerous tissues?

b. What assumption is necessary in order to perform this test?

c. Use a graphical method to evaluate the validity of the assumption in (b).