Which of the Following Factors Are Necessary to Assume There

Which of the following factors are necessary to assume there might be a cause-effect relationship between a particular variable A and a specific illness? (Select all that apply.)

A) It is easy to understand how the variable A could lead to the illness.
B) Research studies consistently demonstrate a relationship between variable A and the illness.
C) The illness is frequently seen without the presence of variable A.
D) The more of variable A present, the sicker the person becomes.
E) Variable A is always found after the person becomes ill.
F) Variable A is never found in the presence of other illnesses.