有关键码值为10、20、30的三个结点，按所有可能的插入顺序去构造二叉排序树，能构造出多少棵不同的二叉排序树( )

A．4
B．5
C．6
D．7

If you often feel tired and the doctor can’t find anything wrong with you, you may be in a state of Subhealth (亚健康). Subhealth is a state between health and illness. Most of the subhealthy people are middle-aged ones who are usually stressed out because of work and family. And some of them are students who are having exams.
If you are subheahlty, it is not difficult for you to become well soon. You should have good living habits. For example, you should get up early and go to bed early. And you should exercise regularly. Exercising can keep you healthy. As for meals, it is better to eat less salt and sugar. Vegetables, fruit and fish are important to your body. You should eat more of them. It is not good to eat too much at one meal, because it may cause unhealthy changes in the digestive tract(消化道). And at last, a balanced diet is very helpful in staying away from subhealth.

How may you feel if you’re subhealthy

A. Happy
B. Comfortable
C. Tired
D. Sad