question: Q:A group of investigators is examining the effect of the drug orlistat as an adjunct therapy to lifestyle modification on weight loss in obese volunteers. 800 obese participants were randomized to receive orlistat in addition to counseling on lifestyle modification and 800 obese participants were randomized to receive counseling on lifestyle modification alone. At the conclusion of the study, the investigators found that patients who underwent combined therapy lost a mean of 8.2 kg (18.1 lb), whereas patients counseled on lifestyle modification alone lost a mean of 4.3 kg (9.5 lb) (p < 0.001). The investigators also observed that of the 120 participants who did not complete the study, 97 participants were in the lifestyle modification group and 23 participants were in the combination group. Based on this information, the investigators should be most concerned about which of the following?? 
  {'A': 'Error in randomization', 'B': 'Lead-time bias', 'C': 'Attrition bias', 'D': 'Nonresponse bias', 'E': 'Confounding bias'},