根据汉语提示完成下列句子，每空一词。

1. Study hard and                           (进步) every day.

2. Mr. Green has many friends at              and             . (国内外)

3. We are good friends, we have                                                    (保持联系)

     each other for many years.

4. Remember not to             rubbish            . (到处乱丢)