cost（过去分词）_______.

The cost of medical care in the United States is very high. The time and money that doctors spend on their medical education is probably one reason for this problem.
A visit to a doctor’s office costs from 15 to 50 dollars. It’s so expensive. Many people in the United States think that doctors are overpaid. However, most doctors disagree. They say that they were asked to study medicine for a long time. Tuition for many years of medical education costs a lot of money. Doctors say that it is necessary for most medical students to borrow money from the bank to pay their tuition. Because their money must be repaid to the bank, young doctors need to receive a lot of money for their work. So they charge(收费)people high prices for medical care.
Therefore the cost of medical care in America is high. Because high tuition is one cause of high costs, one way to lower costs would be to have medical school that are free or have low tuition.
【小题1】The underlined word "tuition" in the passage means “______”

The cost of living in Chengdu is among the lowest in China, ________ the quality of life is probably

one of the highest.

A. since      ????????????? ????????????? ????????????? B. when    ????????????? ????????????? ????????????? C. as      ????????????? ????????????? ????????????? ????????????? D. while

 The cost of living in Chengdu is among the lowest in China, ________ the quality of life is probably one of the highest.

A. since                                 B. when                                C. as                                       D. while