A Cross-Sectional Study Is Investigating the Association Between Smoking and Latent

A cross-sectional study is investigating the association between smoking and latent tuberculosis infection (LTBI) in adults age ≥21 in a rural community.  Interferon-γ release assay (IGRA) results are used to determine LTBI status; subjects with a positive IGRA result undergo chest x-ray to exclude active pulmonary tuberculosis.  Sociodemographic, lifestyle, and health-related information are collected by a standardized questionnaire administered by trained interviewers.  The reported results on IGRA positivity among smokers are as follows: Which of the following factors strongly suggests a potential causal relationship between smoking and LTBI?

A) Higher levels of risk for disease with higher levels of exposure to smoking
B) No residual confounding by unadjusted risk factors
C) Smoking occurred before LTBI
D) Statistical significance of the association between smoking and LTBI
E) Strength of the association between smoking and LTBI