手弧焊时焊接电流的种类根据（）性质进行选择。
A.焊件
B.焊条
C.焊件-焊条
D.任意
参考答案：B
The last half of the 19th century ______ the steady improvement in the means of travel.
A．witnessed
B．has witnessed
C．is witnessed
D．was witnessed
槭叶茑萝一般用（）方式进行繁殖。
A.分株
B.播种
C.嫁接
