20．-Your colleague says she cannot help because she has got much work to do．

　　　 -Well, she ______ say that because she always uses that excuse．

　　　 A．should　　　　 　　　 B．may　　　　　 　　　 C．can　　 　　　　　　　　 D．would

19．A recent study showed how the temperature,______ water freezes, can be changed using electric charges．

　　　 A．at which　 　　　　　　 B．for which　　 　　　　 C．in which　　 　　　　 D．with which