Question: A study is carried out to investigate the association between factor X and disease Z. To do this, 120 subjects with the disease and 420 controls (without the disease) are matched by age and sex. In both groups it is found that 20 subjects were exposed to factor X. What is the odds ratio (OR) between factor X and disease Z? 
 Options:  <1> OR = 1 (no association). <2> OR = 2. <3> OR = 4 <4> OR = 6 <5> OR = 8
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