地震波既有纵波也有横波，纵波和横波在地表附近被认为是匀速传播的，传播速度分别是9.1 km/s和3.7 km/s，在一次地震观测站记录的纵波和横波到达该地的时间差是8s，则地震的震源距这观测站有多远？

Which of the following are problems in measuring unemployment （） Ⅰ. In order to be classified as unemployed, one must be looking for a job Ⅱ. People may be busy but their activities are outside the market labor force Ⅲ. Unemployed workers who are seeking work are included in the labor force along with employed workers

A. Ⅰ only.

B. Ⅱ only.

C. all of the above.