顺次连接任意一个四边形四边的中点所得到的四边形一定是 [ ]
A、平行四边形
B、矩形
C、菱形
D、正方形
答案：A
My friend told me it would be months _________ his family settled down in that city.
A．after
B．since
C．before
D．when
（）利用局部加热的方法将连接处的金属加热至熔化状态而完成的焊接方法。
A.压力焊
B.熔化焊
C.钎焊
D.火焰焊
