--- What a beautiful day!--- Yes, it is _______ that I’d like to take a walk．

If you sleep less than seven hours, you are three times more       to catch a cold．

If you sleep less than 7 hours a day, you are more      to catch a cold.

— Why don’t you want to travel abroad?— Because it becomes ________ expensive ________ before.

However，at times this balance in nature is _______ resulting in a number of possibly unforeseen effects.