In a study of a disease in which all cases that developed were ascertained, if the relative risk for the association between a factor and the disease is equal to or less than 1.0, then: a. there is no association between the factor and the diseaseb. the factor protects against development of the diseasec. either matching or randomization has been unsuccessfuld. the comparison group used was unsuitable, and a valid comparison is not possiblee. there is either no association or a negative association between the factor and the disease