The traffic rule says young children under the age of four and_____less than 40 pounds must be in a

child safety seat.[ ]

A. being weighed      

B. to weigh          

C. weighed          

D. weighing

（）的抗振性较差，变形大，需经时效处理后才能进行机械加工。

A、铸件

B、粉末冶金件

C、焊接件

D、锻件