锡钎焊的焊接条件中，润湿就是表现钎料迅速地流散在整个接头表面，并通过（）反应扩散成为合金属的能力。
A.合金层
B.轩料
C.母材
D.焊接点
参考答案：C
眼球好像一个______，晶状体和角膜的共同作用相当于一个______，视网膜相当于______，眼球和照相机不同的地方在于：照相机镜头的焦距是不变的，而眼球中“凸透镜”的焦距是______的．
I can still vividly remember to pick our steps in the mountain down the deep valley on my 21st birthday.
A．picking ... in the mountains
B．picking ... on the mountain
C．having picked ... from the mountains
D．picking ... from the mountains
