I guess we've already talked about this before but I'll ask you again just _____.[ ]

A. by nature  

B. in return

C. in case  

D. by chance

如果场源是高电压、小电流的源，则场源附近通常电场强度测定值较大；如果场源是低电压、大电流的磁场源，则场源附近通常磁场强度测定值较大。