在生产和生活中，人们常把密度作为所选材料的主要考虑因素，下面属于主要从密度的角度考虑选材的是（　　）

    Tom is the son of a farm owner. One New Year's Day， when he was 15， his father 1 him to work on the farm for one year when he was free. Tom was 2 with his father's idea. "That isn't my job. I have 3 school work to do. "Hearing this， his father said， "I promise (许诺) to give you the best present if you 4 finish one year's work. "Tom thought for a while and 5.

       Starting one Saturday， the boy got up early and worked 6 until evening， just like any other farmer. Time passed quickly. Tom' s crops (庄稼) grew well. 7 the last day of the year， the father called his son to him. "I'm happy to see that you have worked very hard the whole year，" said the father.  " Now， tell me 8 you want "

    The boy smiled and showed his father a big piece of bread made from his wheat (小麦). "I've already got the 9 present. No pains， no gains. I think this is what you wanted 10 to know.” His father was quite pleased to hear that.