4. Complete the sentences with the correct form of the verb:
1 .If you (put) ………………….…….. ice in hot water, it melts.
2. If you started riding your bike to school, you ……………………………… (reduce) air pollution.
3. If you turn out the lights when you’re not using them, you……………………….. (save) electricity.
4. Unless we …………………… (protect) the white tiger, it will become extinct.
5. If he had enough time, he ……………………………….(join) the gym.
6. I will go to the meeting, if I …………………….( have) time.