Question: A study is conducted to evaluate the relationship between lung cancer and exposure to silica. 400 patients diagnosed with lung cancer are selected from the provincial tumor registry and 400 healthy people are chosen randomly from the population resident in the province. The result of the evaluation of this relationship is OR = 1.67; 95% CI = 1.27 - 2.21. It is true: 
 Options:  <1> There is no relationship between exposure to silica and lung cancer. <2> With this study, the relationship between silica and lung cancer can not be evaluated. <3> It is a randomized experimental study with a positive association between exposure to silica and lung cancer. <4> These data show that there is no statistical significance between exposure to silica and lung cancer. <5> It is a case-control study with a positive association between exposure to silica and lung cancer.
5