29. What does the underlined word "margin" Paragraph mean? A. blank B.edge. C decrease D. difference 30. What is the received idea about deep thought A. Sitting more is good for our mental Ihealth B. Sitting is better when we think. C. Excrcise more can improve our cognition D. We should stand while thinking. 31. The passage mainly tells us A. people tend to sit while thinking B. standing more can make our brain healthier C. physical exercise can improve our brain health D. sedentary behavior leads to countless health problems