这个标志是何含义？

A、距有人看守铁路道口150米
B、距无人看守铁路道口150米
C、距无人看守铁路道口100米
D、距有人看守铁路道口100米

根据短文内容，用方框中所给单词的适当形式填空。每词限用一次。

People send letters on horses in the old days. And now people have many ways to send letters. Sending an e-mail is very    81    from sending a paper letter. An e-mail doesn’t need any paper or envelopes. We can “ write” by tapping (击打) the keyboard on the computer. Sending an e-mail is   82    very quick. It takes only a few minutes to    83    to any place in the world. You can send a letter, a picture or    84    videos by e-mail. But you can never send a paper letter or a postcard by it. People all over the world can easily make    85    with each other by e-mail.