为你的上司Denny写一封致歉信给Smith先生。内容如下：
非常抱歉，星期三我无法赴约。因家里有急事，我得离开上海，至少要周末才能返回。我一回来就会给您的秘书打电话，另行安排见面时间。给您带来的诸多不便，深表歉意。

A. birds have to be taught to navigate
B. a bird that has been caged will not fly long distances
C. some birds cannot fly at night
D. some birds seem to follow the stars when they fly at night