Ineed my answer checked!

a scientist is trying to determine whether the use of hair dye contributes to brain cancer. for his study, he compares the medical records of subjects that have been diagnosed with brain cancer to the records of healthy subjects. what is the most significant weakness in this type of study?

a. the test groups must be unusually large to generate significant data.

b. the relationship between the hair dye and the cancer is difficult to determine. < - i think it is this one

c. this type of study is expensive and the data is difficult to analyze.

d. this type of study must rely on medical records, which are often inaccurate or incomplete.