Complete the sentences using suitable clauses/phrases.

1. I could not recognise you at first. It was many years since we _____. 
2. Water is polluted in different ways. It is high time we _____. 
3. I think you are not regular in studies. Be attentive lest _____. 
4. Birds fly in the sky freely. Had I the wings of a bird _____. 
5. There are many obstacles in our way to success. We must work hard so that _____.