【单选题】
_________ the early nineteenth century, people did not use stamps.
【单选题】
When a program has a small number of people_________ it, it is discontinued.
【单选题】
You can_________ yourself to remember better.
【单选题】
You cannot remember things_________ you are tired or unhappy.
【单选题】
Jogging makes the heart stronger and helps people lose_________.
【单选题】
I want to mail this letter to the U.S.._________ is the postage, please?
【单选题】
I asked some policemen to tell me the way home. They smiled and answered me_________ .
【单选题】
You can increase your_________ a quarter or half mile every two weeks or so.
【单选题】
Professor White is out of office right now. Can you leave a_________ or call later?
【判断题】
设b,a1,a2线性相关,b,a2,a3线性无关,则a1,a2,a3线性无关。