The reading of vacuum gauge in the equipment is 100 mmHg, try to calculate the gauge pressure. Given that the atmospheric pressure in this area is 740mmHg, then what is the gauge pressure? 某设备真空表的读数是100 mmHg，请计算一下表压，假设当地大气压为740 mmHg，表压为多少？ ( )

A. -100 mmHg

B. 640 mmHg

C. 840 mmHg

D. -740 mmHg