Mr. Richard hasn’t got any time to play golf these days, because he is ______ in the research of bird language.

American Indians ________ about five percent of the U.S. population .     

At present, computer knowledge has become ___________ requirement as the ability to read and write.

His aunt’s letters ________ of those beautiful days when they used to live together in his hometown.  

If you don’t explain clearly, you will just _______ the reader.