Smile, how warm the world is! It can make anyone happy. It's not like anything else, but just warm, makes you feel warm. Mother's smile is like a spring wind. It's so _ When you are sad, it can make you happy again. When you fail to do something, it can make you stand up and work hard again. When you give up  doing something, it can make you try your best again. Smile, so easy, but so useful as well. Let's learn to smile. Everyone needs others' smile. When we give others a smile, we will feel happy, too. And maybe next time, when you need a smile, the others will give it to you. Let's learn to smile often, I think it can even make the whole world warm. Living with a smile, _ . Which of the following is TRUE?
Smiling is difficult. Smiling is not easy. Smiling is useful. We can't smile every day.