对烤瓷合金的要求不正确的是（）。
A.铸造性能好
B.收缩变形小
C.弹性模量小
D.润湿性好
E.不易变形和磨损
参考答案：C
每一个物体都有它固有的频率。现有一音叉，它的频率是400Hz，表示的物理意义是________________________。它的振动频率会不会因为重敲或轻敲而改变?答：_______。
It ________ nearly two weeks ______ I had received his letter.
A．is, that
B．was, that
C．is since
D．was, since
