Why is(71)fun What delights may its practitioner espect his reward First is the sheer joy of making things.As the child delights in his mud pie,so the adult enjoys building things,especially things of his own design.Secong is the pleasure of making things that are useful to other people.Third is the fascinanon of fashioning complex puzzle-like objects of interlocking moving parts and watching them work in subtle eyeles,playing out the consequences of principies built in from the beginning.Fourth is the joy of always learning,which springs from the(72)nature of the task.In one way or another the problem is ever new,and its solver learns something:sometimes(73),sometimes theoretical,and sometimes both.Finally,there is the delight of working in such a tractable medium.The(74),like the poet,works only slightly removed from nure thought-stuff.Few media of ereation are so flexible,so easy to polish and rework,so readily capable of realizing grand conceptual structures. Yet the program(75),unlike the poet’s words,is real in the sense that it moves and works,producing visible outputs separate from the comstrct itself.It prints results,draws pictures,produces sounds,moves arms.Progamming then is fun because it gratifies creative longings built deep within us and delights sensibities we have in common with all men.

（73）处填（）。

A．semantic

B．practical

C．lexical

D．syntactical

在牛头刨床上用蜗杆副装置刨削凸圆弧时，因刀杆刚性差，易产生振动，故不宜刨削较短的曲面（）