The story is ___87___ a young student. She worked very hard at her lessons. She was too busy ___88___ have a rest. At last, she became ill, so she couldn’t go to sleep. Every night, when she went to bed, she closed her eyes and ___89___ to sleep. But the more she tried, the more she stayed awake. After a while, she went to ___90___ a doctor. “I just can’t go to sleep at night. What ___91__ I do?”

“I have a ___92__,” said the doctor, “Try counting numbers. By the time you reach one thousand, you will be asleep. I am sure of it.” The next day the student returned ___93__ the doctor’s office. “Well,” said the doctor. “___94__ are you today? Did you try it?” The student still looked tired. “Yes,” she said. “I tried counting one, two… up to one thousand. But ___95___ I reached 569, I began to feel sleepy. I had to get up and ___96___ some coffee so that (以便) I ___97__ go on counting to one thousand. But then I still couldn’t fall asleep.”

替代品性价比越（）、用户转换成本越（），其所能产生的竞争压力就越强。

A．低，低

B．低，高

C．高，低

D．高，高