采用口对口吹气法急救时，每分钟吹气（）次。
A、14—16
B、15—18
C、20-22
参考答案：A
You don't have to finish the work today. You can finish it next week.
A. mustn't B. can't C. needn't
客户需求变化有哪三个趋势？
