[单项选择]( )不是焊接缺陷引起的。
A. 产生应力集中
B. 削弱焊缝强度，降低结构承载能力
C. 引起裂纹，造成材料开裂，产品破裂
D. 产生变形

Ours has become a society of employees. A hundred years or so ago only one of every five Americans at work was employed, i. e. , worked for somebody else. Today only one out of five is not employed but working for himself. And when file years ago "being employed" meant working as a factory laborer or as a farmhand, the employee of today is increasingly a middleclass person with a substantial formal education, holding a professional or management job requiring intellectual and technical skills. Indeed, two things have characterized American society during these last fifty years; middle-class and upper-class employees have been the fastest-growing groups in our working population --- growing so fast that the industrial worker, the oldest child of the Indus- trial Revolution, has been losing in. numerical importance despite the expansion of industrial production.
Yet you will find little if anything written on what it is to be an employee. You can find a gr
A. factory laborers will Overtake intellectual employees in number
B. there are as ninny middle-class employees as factory laborers
C. employers have attached great importance to factory laborers
D. the proportion of factory laborers in the total employee population has decreased