1-20 ABCBC ACBCA BACAA CBABC
21-30 ACBDC ABDCD
31-50DDDAD CBCBA ABBDA CCACA 
51.BDDC CBCB DACA ABBD DACA
71. could have finished 
72. being no time to lose 
73. what it is that 
74. will/shall we give in to 
75. What worries many people 
76. how fast the thief was running 
77. before we take 
78. have been looking for 
79. the most widely read
80. seated under the tree

A possible version: 
    It's common sense that we are always encountering much failure in different stages of life. Under these circumstances, what matters most is not only our persistence but also our attitudes.
    In our daily life, however hard we try, failure will accompany us like shades. When it comes to this topic, unforgettable memories crowd in my mind. I once did so badly in a critical exam that I lost a precious opportunity to be admitted to a key senior high school. I wasn’t aware of my laziness and carelessness. Instead, I thought the difficult exam and the neglect of my parents were to blame for my failure. I regarded myself as a stander-by and never learned to shoulder responsibilities for myself. It was my head-teacher's timely instruction that made me face difficult situation any time it appeared, after which I became more confident and diligent. The coming success made me popular among my peers. Never shall I forget this impressive experience.
    It can be concluded from the above mentioned that positive attitude makes a true man while being timid before misfortune results in a failure. Facing any failure bravely can achieve a great success.