下列有关心脏的叙述，正确的是（　　）

阅读短文，下列问题

    If you go into the forest(森林) with friends, stay with them. If you don't, you may get lost(丢失). If you get lost, this is what you should do. Sit down and stay where you are. Don't try to find your friends. Let them find you. You can help them find you by staying in one place. There is another way to help your friends or other people to find you. You can shout or whistle(吹口哨) three times. Stop. Then shout or whistle three times again. Any signal(信号) given three times is a call for help.  

    Keep up shouting or whistling. Always three times together. When people hear you, they will give you two shouts or two whistles. When a signal is given twice, it is an answer to call for help.

     If you don't think that you will get help before night comes, try to make a little house with branches(树枝). Make yourself a bed with leaves(树叶) and grass.

    When you need some water, you have to leave your little branch house to look for it. Don't just walk away. Pick off small branches and drop(扔) them as you walk in order to go back again easily. When you are lost, the most important thing to do is stay in one place.

小明的母亲怀孕了，则她生个女孩的机率是（ ）