起重机用吊钩按构造可分几类？锻造单钩有几个危险截面，分析其危险因素？开口度如何检验？
参考答案：
可分为长型和短吊钩。
锻造吊钩的危险截面有钩颈、钩底和钩心三个截面；
对钩心A-A面：受拉和受弯，有拉长的趋势，会形成开口度增加及扭转；
对钩底B-B面：受剪力，有被剪断的趋势，截面变小而剪断；
对钩颈C-C面：受拉，发生塑性变形。
开口度的检验：测量L，用精度为0.02的游标卡尺测量，和吊钩铭牌上的标注相对照，计算其开口度原尺寸的H值。
| 阅读下面短文，掌握其大意，然后从A、B、C、D四个选项中，选出可以填入空白处的最佳答案。 | ||||
| Robby was 11 years old when his mother sent him to have his first piano lesson. I prefer that students begin at an earlier age, but Robby said that it had always been his 1 dream to hear him play the piano. 2 I took him on as a student. Although Robby tried very hard, he didn't have a talent for 3 . However, he didn't 4 and at the end of each weekly lesson he'd always say:" My mom is going to 5 me play some day. " But it seemed 6 . One day Robby stopped coming to our lessons. He told me that his mom had been 7 and unable to take him to piano lessons, but he was 8 practicing. He asked if he could take part in my concert and I agreed. The night of the concert came. The high school gym was filled 9 parents, friends and relatives. The concert was going well. Then, Robby 10 up on stage. He announced that he had chosen Mozart's Concerto No 21 in C Major. I didn't get ready for what I heard next. His fingers danced on the keys (琴键)…. He played so 11 that everyone clapped (鼓掌) excitedly. In tears, I ran up on stage. "Oh, Robby! 12 did you do it?" "Well, Miss Hondorf... did you 13 I told you my mom was sick? Well, 14 she had cancer and passed away this morning. And well... she was born deaf so tonight was the 15 time she heard me play. I wanted to make it special." | ||||
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