One day,(31).......................... Newton was sitting in his garden under an apple tree, he saw an apple (32)…………………..to the ground. Of course, the fall of an apple is a very common thing. Before Newton,  a  great number of  people had seen an apple fall to the ground, but they had  (33).............  taken the trouble to investigate  such an everyday occurrence. Newton was the first man to ask: “Why does an apple fall to the ground? What makes it fall? Why does it not go up into the sky? Or why does it go sideways when it leaves the tree?”. He thought the matter  for  a long time. It took him many years (34)…………..the problem.  At last he found out that the earth pulls all the things towards its center, and that this was the (35)…………..why the apple had fallen towards the earth and not away from it.

31. A. although           B. because                   C. if                             D. while

32. A. fall                    B. fell                          C. fallen                      D. was falling

33. A. just                   B. ever                         C. already                    D. never

34. A. solved               B. solving                    C. to solve                   D. for solving

35. A. case                  B. reason                     C. effect                      D. method