You are answering a USMLE-style medical question. Question: Please refer to the summary above to answer this question The authors of the study have decided to conduct a follow-up analysis on their data. They decide to stratify their results by CD4+T-lymphocyte count at the time of diagnosis. Among patients with CD4+ cell counts below 200/mm3, cART adherence was a significant predictor of DLBCL risk (RR = 0.52, p = 0.01). However, among patients with CD4+ cell counts above 200/mm3, no relationship was found between DLBCL risk and cART adherence (RR = 0.96, p = 0.36). Which of the following explains for the difference observed between the two strata?" Options: A) Poor generalizability B) Stratified sampling C) Random error D) Effect modification Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.