下述哪一项改进不能节约材料（）。
A.搅拌浆实心轴变为空心轴
B.矩形梁变为工字钢梁
C.圆形梁变为槽钢梁
D.槽钢梁变为工字钢梁
参考答案：C
我国的耕地，北方以__________为主，南方以__________为主。
Computer hardware engineers research, design, develop, and test computer hardware and supervise its manufacture and (73) . Hardware refers to computer (74) , circuit boards, computer systems, and related equipment such as keyboards, modems, and printers.
A．chips
B．frame
C．structure
D．cell
