— Thanks a lot, Mr. Wang. I don't know how to ________ the help you gave me during my stay here.

— It's my pleasure. Welcome to China again when you have time. [      ]

A. pay off  

B. pay back  

C. pay for 

D. pay to

应用尿蛋白监测运动训练时，应注意该指标仅适用于群体间前后相互对比。