PTN是指（）
A.分组传送网
B.同步传送网
C.异步传送网
D.波分传送网
参考答案：A
Last night I _____ the homepage for my class when my friend called me. [ ]
A. designed
B. has designed
C. was designing
D. would design
熔焊时，由焊接热源输入给单位长度焊缝上的能量称为（）。
A.碳当量
B.熔合比
C.焊缝成形系数
D.热输入
