【名词&注释】
灵敏度(sensitivity)、水准器(leveling instrument)、精度高(high precision)、观察窗、四等水准测量(fourth-grade-leveling)
[单选题]四等水准测量(fourth-grade-leveling)中,黑面高差减(红面高差±0.1m)应不超过()
A. 2mm B. 3mm C. 5mm D. 7mm