连续焊缝
参考答案:
连续焊接的焊缝。
There _____ many kinds of birds around here,
but now we seldom see them because of the air pollution. [ ]
A. use to
B. use to have
C. used to be
D. are
蝇属于昆虫纲的()
A.鳞翅目
B.双翅目
C.虱目
D.蚤目